1) Some sort of internal infection
2) Hoof problem
3) Hock problem
4) Cycle/hormone problem
Based on my knowledge of Izzy, I would say that I think #4 or #5 is most likely. An infection is possible, but I would expect other symptoms, maybe general droopiness, dull eyes, more discomfort. Her feet look good. It's not just me saying that--every farrier I've ever had work on her feet (4 different ones) thought she was pretty ok, especially in the back. She's sounds and moves relatively well, which is also why I doubt it's a hock problem. She's sound and even. She is genetically predisposed to be sound (yay!), she didn't work hard as a young horse, and I've brought her on slowly. Those options are possible, but I'm not thinking they are most likely.
Which brings us to hormones and ulcers.
Izzy never gave me any trouble in her cycle until her last one last fall, which I mentioned in the appropriately named post, PMS Pony. She is also 7, almost 8, so she is in a prime time of life to be having these problems. Since her last cycle was rough, it's definitely possible that her first one this year is correspondingly rough.
The other obvious possibility which I had overlooked is that her ulcers are acting up again. Hm... this one strikes a chord with me. Specifically, Izzy's ulcer problem started last summer when she was confined to her run except for a few minutes a day when I was able to turn her out in the arena. We're at a much better place now, but all the rain and snow and yuckiness has seriously cut in to available turn out time. It could easily be that this nasty lil' problem is cropping it's head up again.
Ok, our current short term plan is as follows:
1) switch to the loading dose on her ulcer medication
2) try to maximize turnout as weather allows
3) tack change: ride in the western saddle outside the arena at a walk to see if there's an element of being arena sour going on--maybe she just needs out
4) continue to monitor for any signs of cycling. Her back legs were a little messy two weeks ago, but I haven't noticed it since and she hasn't been presenting herself to geldings that I've noticed.
5) I'm considering having a massage therapist out to work on her. That would maybe indicate any more specific pains and would rule out a serious personality conflict, which I don't think is the problem, but would cover my bases.
Any other thoughts? I'm thinking we'll stick to this for about a week and look for improvement. If she's still acting weird, the next step will be to call a vet, but that's expensive and my faith in conventional medicine is only so-so.