What is going on with mare's sudden bad behavior?

Her background..
4 y/o ottb
She was bought in august by a green owner with his girlfriend leading his as well as the mare's training.
Something happened and made her incredibly back sore
4 months stall rest
During that time they broke up and the owner was left with a relatively green horse and no idea how to take care of her
Stables owner contacted me and asked me to work with her.
So!
Everythings going great, she's underweight and undermuscled so we've switched her from oats and sweet feed to gro n win with beet pulp and a lot of extra hay.
Had he feet trimmed
Went on a trail and she was great.
Then about 2 days ago she starts acting up. Sidestepping, trying to rear up just in general not listening.
I'm trying to figure out what it is.
Maybe she's sore from doing trot work? We did do a lot of walk trot the days before she turned sour.
She's in season..could that explain the snottyness?
Could it be coming off the sugar feed? Like a withdrawl?
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Perhaps with better nutrition and proper care, this mare is feeling good and testing you a bit.
Also, she is green broke. Maybe she is seeing what she can get away with. It's possible she picked up some bad habits from her inexperienced owners.
Has she had her back checked recently?

^This^, & you may've done too much walk/trot for her back.

I've been wondering about the proper amount of trot. I rode her for about 30 mins and about half of it was trot working on bending. That sounds about right that she would be testing..I'm going to show her owner proper respect games and just general this is ok bjut don't let her that.
Her back was checked in late september I believe. There's an equine massouse on site.. I could ask her to check her out I think its 50 for a massage the only problem is she's not my horse so I can only tell her owner what's best and hope he follows through.
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I had a similar situation with an OTTB mare that had just turned 4 when I got her. She was laid back and sweet......until she was fed (was very thin and malnourished before). So, she may just be exposing her personality now that she feels better. However, with her history of injury coupled with the fact that this happened right after the trot work, I personally would check her for pain first. You may also want to check the saddle fit.

How is she on the ground?

Best luck in finding the problem!

That's the thing she doesn't show any signs of pain like by pressing on her back, currying, brushing, nothing makes her flinch or pin her ears.
I'm thinking it is the malnourishment.. if she doesn't turn around attitude wise I'm going to talk to him abou selling her back to the stables pwner and getting a different horse..if this IS her personality then he needs something a little easier.
She's fine on theep ground the only issue is she doesn't lunge to the left. It's something we're working on.
In the arena she's perfect. I've never had issues with her. Its when we were leaving for a trail(which she's been on plenty of times) we got to the end of the driveway and she just wasn't having it. We weren't alone. My boyfriend was riding my gelding(whome she's been down the trail with before).
I was told she rode great last night but when her owner was taking her bridle off to let her roll she took off and basically drug him around the arena. I wasn't there I can't testify, this is just what he texted me.
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She is green, so that means she isn't dead quiet. You should expect her to have the occasional fit about something she doesn't want to do. Sounds like a normal 4 year old horse to me.

Racehorses go counter-clockwise around tracks, meaning they are normally bend to the right. Since she was previously race trained, of course she is going have problems going to the left as this is something she has never done before, and probably has balance issues.

Wouldn't counter clockwise be to the left? That's what's odd is she doesn't want to lunge to the left but when asked to canter to the right she literally points left while cantering to the right. SheMs very confused but that's something I've been expecting. I just didn't know what was causing the sudden attitude issues but I think its been narrowed down to better nutrition making a frisky horse.
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Ahh...my mistake. I was thinking backwards, so that's my fault.

Does she get angry if you touch her belly? My mare went from completely fine to psycho-mare-ish behaviour in the course of two days due to ulcers. I noticed it only because I went to cinch her up and brushed the middle of her belly and almost got my hand cowkicked off.

And being stalled for 4 months could likely cause ulcers. When my vet came out to check Abby, she asked if she was stalled or grained. Abby got neither, so we're not sure how she got it. And when I tell other people about her ulcers, they ask if she's stalled often.