I would think an important question to get answered is WHY there was cum at the base of the condom. Why did it leak?
Did he apply the condom correctly? Did he leave unfilled space at the end of the condom to accept and hold the ejaculate?
Is the condom the right size (yes, there are different sizes) and does it grip or hold the penis enough not to slip?
The fact that there may have been some cum next to your vagina opening does not equal pregnancy... but it is enough to raise concern.
The fact that you have started BC pills when this happens makes it doubly doubtful that you are or will become pregnant from this instance,
but both you and your boyfriend need to review how to correctly wear and use a condom...including the use of water-based lube if necessary.
Good luck.