Just because you've had the same tack for 2 years, doesn't mean it fits her NOW. And using a different saddle wouldn't alleviate pain that is already there. Similarly, running around in the pasture doesn't mean she has no pain. I would seriously get her checked out for pain and have a professional check the fit of your saddle.
My mare had a saddle that fit, I worked her more and built up her topline (back / shoulder muscles), the saddle started to pinch and she started to buck. Then she started to come up 3 legged lame. When I'd strip her tack off and turn her loose she would run away bucking.
Turns out the saddle no longer fit. I got a new one and had chiropractor adjust her, and VOILA - problem fixed.