Someone made a 5$ purchase on my account

I was away over the weekend and upon returning home, i noticed someone had purchased Haypi kingdom (free) and made a 5$ in app purchase using my account. I am at a loss here and have no idea what i should do. I already changed my password, but I need to find out how to fix this. When i click report a problem in itunes, the apple support website just pops up and nothing else. Anyone have the same issue!?