Semen isn't the only thing that you have to worry about either. Have you ever been having sex, him pulled out during, to switch positions or what not, if so, you may have noticed that there was already stuff in the condom, pre-cum.
When a condom breaks, that pre-cum goes inside of you, and then that "little" bit of semen goes, when he goes ahead and continues.
Guys are assholes, when they say,"i didn't know the condom broke" because, yes they do, they are able to feel everything with a condom on or off.
Anyway, I think that you need to take a morning after pill, if you can get your hands on one. Or consider going to the doctor.
Why aren't you on birth control of some sort?
Good luck.