Yes, sperm can sometimes leak out of the opening of a condom if, for example, your boyfriend leaves the condom on after his erection goes.
It sounds like it's possible that some sperm leaked out of the condom and into your vagina. In the future, it's always a good idea for the guy to.
Lots of questions, like: Can you get pregnant using a condom even if it doesn't break? And: Can sperm leak out the base of a condom? Yep.