Your period could just be late, or that could be an early pregnancy and your one of the few that has vaginal bleeding in early pregnancy. good luck!
It could be any number of things. You could be just having a weird/off cycle. You could also have a cyst or something like that. I also have to say, you might be pregnant. The fact that you were late and didn't have your normal period and are now cramping, it's a possibility. Definitely see you're doctor to rule out any other issues (like the cyst) but maybe take a pregnancy test in the mean time, just in case.