Hmm, I don't know. Her back could be out of alignment or she could have a slight roach back that has just recently started to bother her. Or she could be getting arthritis in her spine, I just don't know. Probably the only one that could make a for sure decision is the vet but after reading your other post, that may not be an option. If it is possible, you might give her a break from riding for a couple of weeks and see if there is any change in the tenderness, size, or shape of the lump and go from there.