She's not coming back into heat at the shows, but she is getting real excited and tensing up in her back when she squats and pees. So, if she's tensing up in her back that's why she's getting tender.
I'd do a few things to see if I could fix it.
1) Find a chiropractor to make sure she isn't out somewhere in her back
2) Use a nice big gob of Vicks in her nose before you put her on the trailer next time, it will deaden her sense of smell and it may help her to not get so excited
3) Warm her back up good before you try to ride her. Then you're going to have to get on and work her until she's not tense/sore anymore before your classes.
Be ready for her to buck and carry on if you don't scratch her next time, you've taught her to tense up and not be ridden at shows, she's probably going to object next time you want her to work.
Regumate is not the answer, training and making sure there's no physical reason to not ride her is. Regumate costs a fortune, it's very dangerous for humans to handle and it's way over used. In addition to messing with your hormones, Regumate is a known carcinogen and you need to be very careful when handling it.