A Couple of things;
1> Ulcers usually have a bunch more signs than just Girthy.
2> Saddle fit etc can be the cause but usually its not.
3> Back out etc can be the cause but it will show up in the gait and stride also.
So to me it sounds alot like "I don't want to." Riding means work and work it is. Willingness to do the work is one of the three things that are usually the underlying causes of most issues.
The other two are respectful and trying.
All mares go thru the "I don't want to" syndrome. Kiknd of goes with the gender. Lots of ways to work on it but if I has already goten to aggression--and biting is definitely a form of aggression-- then it seems like there are some respect issues also.
A suggestion: put her in the middle of a round pen and try to saddle her. If she shows any sign of aggression or "I don't want to" move her feet--and I mean move her feet for a while. Ask her to stop and then try again. Same thing but move her feet longer and longer until she realizes that the best way out is for her to stand like a lady and accept the saddle. Kind of like-- you are going to work anyway and the work is much harder when you act like a """.
Lots of positives to this-- Human controls horses feet, horse will do the work with or without the saddle, Human is the leader--horse is the follower, etc.
All three of these things are wrong.
If you believe they are true, please back them up with references.
Ulcers CAN have many signs. They can also just have one out of ten. Some horses are reluctant to go forward. This can be (linked, SCIENTIFICALLY) traced to ulcers. (The acid in the stomach sloshes forward into the more sensitive regions of the stomach where the cells are not protected. This hurts.) Some horses don't eat. Some horses have NO symptoms except 'eh' and have major ulcers. Do not denounce something you do not understand well.
Saddle fit is a MAJOR, MAJOR cause of back pain and horses being snarky about being saddled. ESPECIALLY if they start to get worse for no reason. The behavior does not reward itself--the mare is bad, and the OP rides her anyway. There is no REWARD to make the mare get worse, which means it is almost ONE HUNDRED PERCENT pain related. Please look up positive and negative reinforcement. There is absolutely no reason in the world for the mare to get worse UNLESS the OP started putting her back in her stall/pasture instead of working her, which I seriously doubt.
Also, why would the mare be crabby about grooming if she just 'didn't want' to be ridden?
And lastly, back soreness almost never shows up as lameness. It shows up in lack of range of motion, stiffness to one side or in general, a guarded way of moving (which is hard to diagnoise if you have always seen the horse move with back pain), CRABBY TO BE SADDLED, etc. A personal friend of mine had FIVE saddles to try on her mare and she would dance, move around, and bite. She got a saddle pad raiser to lift the saddle off the mare's withers, and imagine that... NO more crabbiness.
To the OP:
I highly suspect ulcers.
Your mare has muscling and a 'tucked up' look to her abs that suggest ulcers. Could you post a picture of her standing square?
You may not have to treat ulcers with medication. Is she in a stall? Get her moved to pasture right away. If this is not an option (Although it is the very best for her), then make sure she has food in her stomach at all times. Horses produce acid in their stomach whether or not she is eating. (Humans only produce acid when we eat.) If she does not have something to soak up the acid (hay, grass, etc.) she almost 100% will get ulcers. This is why pasture is good.
Also, I would try feeding her hay as you groom her and tack up. When you ride a horse on an empty stomach, the acid that the stomach produces without fail is just jumping around in her stomach the entire ride. You could buy a cheap anti-acid supplement and see if it makes any changes--if so, she definitely has ulcers and then we have a range of ways to treat it. Keeping food in front of her, whether it be grass or hay, will also heal it, but it will take much longer.
Keep us informed!
(Clearly my horse has had ulcers before so I am not talking out of my you-know-what. ;) )