Horse acting up on short ditch-like downhill

Hello! Not sure if this should go in the health or training section...
So background first: :) Feel free to skip ahead to the bold, sorry its so long, I try to cover all possible questions!
I ride my 3.5 yr old a few times a week all near our property- which is all rather hilly, a decent amount of rocks at times, and some flats. Hardly every do we ride at more than a walk, unless its on the flats I know are clear. She has gotten very used to riding on this terrain and does so with no issues at all anymore.
So this morning we went hacking off the trail, in nice clear pinebed type ground. Some up, some down, some flat on small plateaus.
So the reason for my question:
Today when we were hacking randomly as mentioned above, on a small wide ditch-styled downhill, she started tossing her head and doing some big crow hops, then sped up a bit, and stopped when I gained control. I stopped, we checked her over, thought maybe it was caused by being scared of the travel mug I had hanging off my cantle- that it bopped her in the butt a few times and she thought it was a fly, so I took it off. I had never hung it there before.
Ride some more... all mostly uphill or flats, nothing. Then go to go down a rather wide semi-steep, but super easy for her, and she does the exact same thing, I catch her mid hops, try to force her straight down, to test if its behavior or injury. She refuses to go straight down- instead she goes down and left at the same time, skirting the incline. To be doubly sure its not behavioral, I reverse, get back up it, and go down again- nope, no go, same exact behavior. On that one I got off again, checked her saddle and body over one more time, saw nothing, and hopped back on.
I led my daughter on her for about 15min on areas I knew were flat or only slight uphills. She behaved perfectly, and didn't appear lame or bothered in any way.
She did the downhill head toss/ crow hops 2 or 3 more times on the exact same type of ground on the way home- these with no rider and in spots she has ridden in many times without issue.
Checked her over really well without the saddle when we got home, couldn't find anything- no swelling, etc.
Anyone have any ideas? My only thought, which I just had, lol, is that the saddle creeps forward just enough to bother her? I already ride in a breast collar, and am going to buy a crupper, just need to pick one out, and don't have a clue how.
I will still post my question, on the off chance there is something I am missing.

Thank you if you made it to the end and had anything to contribute!
Anja and Sonata

It could very well be the saddle slipping forwards and putting pressure on her withers. Has the saddle been professionally fitted?
Saddle fit was my first thought even reading the title - common problem with ill-fitting saddles.
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It wasn't been professionally fitted, but was fitted by my old trainer who really knows her stuff. The more I think about it, based off of little tiffs she has had when the saddle has slid up on her in the past, I am 99% sure that is what the problem was. And for some stupid reason I had girthed her on the next to normal hole, making it a bit looser than normal too! Poor girl, lol.
Thanks for the reply!

That behavior might also be indicative of some sort of structural issue.

Based on my own experience with one of my horses, that nearly identical behavior was brought on by a fractured sacrum

I know it wasn't saddle fit because I have been riding bareback my entire life and this particular horse is so big barreled that I didn't have a saddle to fit him, anyway.

I know what happened to my horse but it's entirely possible your horse could have hurt itself just being a youngster in the pasture.

It's a toss up as to whether you want to spend $$$ on X-rays, or get a qualified chiropractore to look at the horse.

Were it my horse, I would opt for the chiro but again, somebody well qualified and with good references

Another thought I just had: She's 3-1/2. What's the condition of her teeth? Are all her caps off? If you're using a bit, it might be possible that, somehow, the bit is hitting her teeth wrong. Maybe her teeth need some help? Maybe the bit isn't adjusted right?

I dunno, that thought is called "grabbing at straws" but still, something to double check and be sure about

It could also be that she just hasn't learned yet, how to go up and down hills. I was raised in the flatlands of Ohio so when I stared hard trail riding all over everywhere, I had young horses, that I literally walked down a few hills with them, so I could "whoa and go" them from the ground in a way that would teach them to walk straight on and keep their butts under themselves.

Thanks walkinthewalk!
Nope, the hills def. Aren't it, she came from flat SC with me, so she had issues at first, now she can walk them like a champ!
She did lose a cap a few weeks ago, and I have been checking teeth over here and there, to see for more- all looks good there. :)
I am still waiting for a 3.5 yr old youngster pasture moment! LOL This girl seems beyond her years, I tell ya. :)
At the moment, based off previous behavior when going down hill with the saddle I didn't tighten all the way properly (stupid me lol) I am going to chalk this one up to having not gotten the girth tight enough. I am def. Going to keep an eye on her beyond that, but she doesn't act lame or wonky any other way.
I am researching cruppers right now. Not sure which to buy. Leaning towards the Zilco Endurance crupper... if anyone has thoughts on it. :) (I did put a post about cruppers in the tack section, but no bites yet!)

Thanks all!