What happened to my skin?

I'm not entirely sure, but it looks like your skin uses a second layer. In 1.8, a second layer was introduced, and because of that the skin format had to change. In the place where 1.8's skin reader would have decided that was the overlay, 1.7's skin reader displays the top layer as the only one. You'll have to switch to 1.8 to use that skin.

As of 1.8, there is a new player model framework: the "Alex" skin. It is a little slimmer than the "Steve" skin, and I believe it is supposed to be more feminine.

From what I can see of your skin, it looks like it is intended to be a more feminine skin. If the skin is relatively new, it was probably made for the "Alex" model, not the "Steve" model which you will have if you are on 1.7.

I suggest either finding a new skin or updating to 1.8 and changing your preference to "Alex" on the Minecraft website.