Why is my 2008 Pontiac G6 burning gas really fast

Im filling up my tank every two-three days, and I barely even go anywhere. They told me I had carbon build up I got that flushed but im still burning gas like crazy. My tires are new, I get oil changed on time, I dont drive fast, so I dont know what the problem is. They also tried changing my gas cap because it was always saying check gas cap after I put gas in my car.