This summer, before we found my current TB, I tried out a 17 hh TB (also an ex-racer) who had a bump like that...I didn't know what it was at the time; I figured if we decided to go further with him the vet would know if it was a problem. We decided not to buy him, but it didn't seem to be affecting him...I did some flatwork, putting him into a fairly collected frame, and jumped about 3 feet that day, and he didn't seem to have issues. But thoroughbreds seem to be especially sensitive to any pain on their backs...mine is very cold-backed when mounting (he has bolted and bucked me off when I was halfway on before); I'm hoping when we get a saddle that actually fits him (maybe for the first time in his life) it will fade or go away completely. We are trying out a Black Country saddle right now that he really loves and is coming rounder and softer than before. Maybe your mare is reacting to her tack as well?? What type of saddle do you have on her? I think in general TBs and other hotter horses react really well to saddles with wool flocking, I.e. Stubben, Clack Country. It is just so much softer and more forgiving on their backs. We are debating between two saddles Sprite really likes, a Stubben genesis and the Black County. He is so much quieter and softer in those; I have put a Collegiate, Amerigo, Stackhouse, Antares, even a Devoucoux on him, and he likes the Stubben and Black County. He is very specific about even the model; he hated the Stubben Edelweiss, which is only slightly different from the Genesis, because apparently the panels weren't shaped quite right and it moved around too much on his back. So sorry to ramble on here...I'm just thinking if her jigging might be related to her saddle. But I would definetly look into getting the chiropracter/vet out to check that bump...I think that would be the primary cause.
He knows when you're happy; He knows when you're comfortable; He knows when you're confident; And he always knows when you have carrots.