Interestingly enough, many digestive issues are noted as causing back pain of some sort. Gall bladder, for one and many others.
A doctor’s diagnosis over the phone isn’t the way to go about solving this problem. In fact, a GP is no way to go about solving this problem. For the most part, they just aren’t trained to deal with this sort of thing and tend to classify most issues as a bug, gas, diet etc and don’t really look too deep until the problems get to the point that they are past being easily remedied.
To start, though, you need to get your ass into the doc’s office and explain the symptoms to him/her properly. Explain your diet in detail- what you eat, times of day, when does the pain occur, is the pain episodic, where exactly does it hurt etc. They will possibly send you for basic stuff like x-rays, blood tests etc. and perhaps a hida scan to check your gall bladder function. If that doesn’t uncover anything, he may pass you along to a gastro intestinal doc who may follow up with further blood tests, a lactose tolerance test, wheat allergy test among others.
Look, it may be indigestion, it may be something else, but you need to get a proper diagnosis. Get to it and rest and scale back the work outs until you understand the problem. Many problems can be made worse by hitting the gym hard. Trust me. Keep us all posted, too.