Chiro/vet is coming out Friday to rule that out. I just came inside from lunging her & I trotted her in-hand down some small slopes. She was rambunctious but not tripping. Her canter on the lunge definitely needs work, but I really think thats a strength issue vs pain as I watched her move. She was using her back well, she's just so long-legged & it's just hard for her to hold everything together on a circle like that.
Based on some of your great suggestions, I think I'm going to a) rule out pain with the chiro/vet, then b) start upping her work # times / week with a lot of in-hand work on transitions and bring in ground poles, cavalletti to have her strengthen that back & push through the "momma, i dont wanna / cant" antics. I think I found an indoor nearby to trailer to for when its too wet here at home.
Like I said, thank you all for pointing out different things from your experiences that I may not have thought of!
Another option is making the investment and sending her to a trainer for a 30-60 day boot camp. I don't know that she's in enough work for a vet to see anything, and I think I chiro is a waste of time. With a 4 year old IMO the only useful tests would be xrays and neurological exams. You need to get her in serious work and see if it either escalates or gets better, as well as have a regular set of eyes on her that you trust.
I had a 4 year old mare I bought that was called sound by vets and chiros for years, but had some weird issues under saddle - she had mild neurological deficits and a chip in her stifle. I currently have a 4 year old who can have bratty outbursts. The difference is pretty easy to see once they are in work.
I'll just chip in to second considering hormones/diet. My young mare (5 this year) was really dealing with awful hormones last year! Even the young horse trainer noted she seemed to be in heat every other week, and she'd get kind of 'stuck' and gassy. I considered Regumate, but I have her at home and am not willing to risk exposing myself to it. Based on someone's input here (ChocoMare maybe?) I removed all soy from her diet. No more extreme heats and she seems much more comfortable going forward! She's not a hard keeper, so just gets Equipride with a bit of beet pulp, rice bran, and oil.
Had vet/chiro out this morning. She has been condemned to at least 3-4 months off. Said her hind end was really out of whack most likely due to a pasture injury (fall, slip, or something). Didn't think the riding I've been doing would have caused that much. Though having a longish, weak back may have compounded it. Legs & stifles tested fine , so though he said 90% of times, back soreness comes from there, he thinks she's in the 10% with a true acute back injury.
Not quite the news I was hoping for... but we'll recheck & re-adjust her in a month and go from there. Lesson to be learned--go to your vet first to rule out injury/pain/whatever even in the young ones who've had easy lives!