Could this be due to her wearing her blanket when the temperatures aren't cold enough to warrant one? I prefer to have her be "naked" if it's over 50, 55 at most, but yesterday she had her sheet on in 65 degree weather when I came out. The barn help tends to either not blanket enough when it's freezing, or will leave them sweating in layers when they don't need anything...my issue with them is a different story. I feel like I might be over-thinking this going to her being blanketed unnecessarily as a culprit.
Now, other factors are that I recently took her off the 24/7 pasture as it had done wonders for her soreness over her back and she was getting too plump. I put her on stall and started her on a handful of grain to go with the joint supplement she's on. She has been on the loading dose of the supplement, which is also for hoof and coat.