I am not saying this with any absolution but it is possible that your teeth are demineralizing. There is also a condition called dental fluorosis, which could be a side-effect from the fluoride in the toothpaste you use. Fortunately, they say it is only a "cosmetic" issue, and not necessarily an actual health problem. When they remove your braces, you will also undergo a cleaning which should get rid of them. Don't take it from me though. They are your teeth after all.