I asked around today, but it seems more information is needed. A pure guess was that maybe the saddle is too small. It may pinch her only when you are sitting in the saddle. She may have gained a few pounds with her winter's rest or she may not have lost her winter girth yet. Which was my guess.
My DD's horse, Lena, does that. When winters are mild and she can get some riding done with Lena. But no matter how much she exercises her horse, she keeps that winter girth on. Then in the spring, it miraculously disappears.
Did you mention her age? Sometimes a horse will physically mature at about 7 or 8. Our horses were 5 when we got them. They are now 8 and we had to buy a saddle for Lena this year. She is a cutter, too and DD also trail rides with her. She muscled out of her saddle.
Anyway, just some thoughts on the subject.