I keep getting flats. What's wrong with my tire? My back tire caught a flat from a pot hole. The next day I take it to a shop and I get an inner tube change. From the shop I ride the bike home, but the tire catches another flat on the way. What do I do now? By the way I have a 52cm pure fixed bike
It could just be bad luck.

Or, there may be a piece of glass, a tack/nail, or caltrop stuck in your tire in a way that wasn't visible to the shop that replaced your inner tube. It may be deeply embedded in the rubber tread, invisible on both sides, and only pierce the inner tube when under both air pressure and the weight of a rider.

When you or the shop changes the inner tube yet again, they should keep track of the orientation of the inner tube to the outer tire and where the puncture is. They should triple-check the outer tire casing at the location of the puncture. 

Or it could be bad luck.