Tooth pain when eating chocolate, but not a cavity?!

A month ago, I noticed I'd get a toothache when eating chocolate, around the area on a tooth where I have some decalcification. I bought a flouride rinse and stepped up my hygiene, and it no longer hurts when I eat chocolate. It also stings when i have very cold water, but again, it does less so now that I'm using flouride daily. Now, I was at the dentist just an hour ago, and it hurt when she cleaned that area with the hook, but she found no cavity. Her advice was just to keep an eye on it and we'd see at the next cleaning.

Any idea what the problem could be?