If your saddle truely fits properly, and she has had her teeth floated with in the last 2 years, and she has nice big shoes on. Then here are a few more suggestions.

She is at the right age to have her hock joints start to fuse. This can be a very painful process.

Also as far as her heat cycles go. A mares cycle is about 28 days but they can come into heat and ovulate every 14 days. You may want to track her cycle a little more closely. It could be that one of her ovaries is more painful then the other when she ovulates but they are both uncomfortable.

The biggest thing is that you need to figure out wether it is behavioral or physical, once you determine what it is, you can go from there.

You may need to have your vet do a good "lameness" exam with flextions and maybe even ultrasound her ovaries if nothing is positive on the lameness exam, (this way you know exactly where she is in her cycle to keep track). That way you can rule out if it is physical. Once you talk with your vet you could also ask him about using a bute to see if it helps.

Had a mare once that had painful heat cycles, at first we were not sure what was going on, her lameness exam checked out fine but we were still not sure what it could be. So we decided to try using bute to help diagnose. We would bute (a low dose) her the night before and the morning of a ride to see if it made a difference. We found that it did seem to help, so we went back to the drawing board by this time it was late spring and about twice a month she would act cranky when she was ridden, then we noticed that she started to colic mildly about once a month, but would be fine with a little banamine. We decided to ultrasound her after a colic episode and found out that she had just ovulated. In her case her right ovary causes her considerable pain at ovulation causing the colicky episodes but in general she would have uncomfortable ovulations on both sides.

Hope this helps but like I said check her physically first if it is not that then it is her behavior, and that will take some training. Good luck.