It is more than likely the design of the shoe. The front part, where the arch meets the back maybe a little longer. May I ask (and I don't mean to pry) where did you purchase these shoes? This has happened to me. It happens with very few labels, but I had purchased two pairs of Charlotte Russe shoes (pumps) and I noticed that my heel was flipping out the back. My ex boyfriend bought me a pair of Dior pumps and they fit great. The heels are 6' on the Dior pumps, whereas; on the Charlotte Russe pumps they were only 3'. Recently I found some affordable pumps at TJ Maxx and they fit my feet amazingly and they possessed a 5 and a half inch heel. I noticed that the arch was balanced. If you notice the front looking smaller than the back and there is more room in the back, you will end up with your heel flipping out. This can also happen if the width is actually bigger, as well. If you use a gel cushion, the shoe will not only be too tight, you will endure pain, as a result, as well as, stretching of the shoe and fabric.