I agree with Loosie, it sounds like she needs a though check up by a vet. How long have you been riding for? What was she used for previously before she was put out to pasture for a couple years? It sounds like she might have had some issues before she was put out to pasture that might have just strengthed since she was let out and not ridden alot. My one mare has a real sensitive back and we have to be careful on what saddles we put on her. It might have been from past saddle fitting experiences gone wrong(she is also seventeen), who knows.
But I agree with all of the above, get her checked out to make sure nothing is wrong with her body structure, teeth, hormones, ect. Then move on from there.