My vet came out to do a repro exam on my young mare. I have been having issues with the canter under saddle and my vet thought it might be good to take a look. She found that her right ovary was enlarged (left was normal sized) and there were a lot of follicles. She said that her right ovary was cycling "as if it were April" (and it was early January). We also took a blood sample but didn't find anything remarkable in the results. She also determined that this mare cycles year round (oh joy!)
My vet said to put her on regumate (12 cc/day). She said that you usually see an improvement in less than a week. It has been 10 days now and my mare continues to act as if it is very painful for her to canter under saddle.
As an FYI--she has been looked at by another sporthorse vet who checked for back soreness (none was found) and did chiro on her. He thought her problem was ulcers so she also has been on ulcer meds for a month. No effect. Her feed was changed to a low carb/high fat feed about 6 weeks ago. I have a massage person who works on her monthly and thinks her back is in great shape, her topline is looking wonderful. Saddle fitter has also been out to make sure her saddles fit within the last couple months.
Your mare may be retaining a large follicle and may still have lower-back pain as a result. Did your vet also give a shot of oxytocin (I think that's what they use ...) to cause her to release any retained follicles? It might be worth a 2nd ultrasound to be sure.
"You become responsible, forever, for what you have tamed." - The Little Prince
Try some magnesium, a deficiency (which is common) is often responsible for sore, tight muscles. PerformanceEquineUSA has a very digestible form that has made a difference for my horses. I'm also using the human form and my aching back is sore no more! Great stuff.