Ok,ok so I had this same exact problem with my betta, and I found that he was just bumping into things. Don't worry, it will grow back. -though I must warn you, it will be a slightly difrant color and will take several months to grow back- my little Sparkey is very reckless and tears his fins and bumps scales also. But everything will grow back with time.
Mine did this twice. He jumped up when I was about to lift the lid to feed him and he scraped a lot of scales off. It freaked me out cause it was bloody, but its healing nicely and his scales are growing back.