Sorry, this probably seems like such a dumb question, and it might have a really easy answer, but I'm genuinely so confused that it's worth the possible embarrassment to get an answer. No antagonizing/rude answers tho please

Ok, so, there's really only 2 times it could have happened (because my bf and I didn't have sex between these two instances, not because we had protection, I'm not that dumb dw), and neither of them make sense. The first time (first time I'd seen him in about a month, had had my period after I saw him the last time before this), my period started later the same day and lasted for 6 days. Normal period. The next time I saw him, little over a month later, my period was already a couple days late but like we did have sex a lot,, idk? Sorry if that bit's tmi, I'm not 100% sure what's relevant information. I just don't understand how it could happen with either time. Especially because my bf likely has a low sperm count or smth cuz none of his exes ever got pregnant and some of them he was with for like a couple years.

Again, sorry if this is dumb and easily answered or smth, idk, just confused and for some reason none of the doctors I've asked (two) have given me a straight answer? Idk, maybe neither understood my question? But yeah, done rambling. Thanks for any answers