So my boyfriend and I were about to have sex but we kind of didn't. He just put it in (not even all of it just like the head) for like 5 seconds nad is pretty sure he didn't pre-ejaculate or inside me. My period is late right now and I'm really nauseious right now (idk if its cause I'm stressed out and nervous) but do you think I could be pregnant??

Yes, you could be pregnant.

There's more than enough sperm in pre-ejaculate (if his penis got hard pre-ejaculate was present) to get you pregnant. He didn't even have to go in, he just had to be near your vagina. The fact that he went in, even if it was just the head, makes it even more possible.

You'll have to wait until your period is at least 2 weeks late, then test using first morning urine. If that test is negative, wait until your next period and if you miss it, test again or get a blood test.

If you're not ready to be a mom, don't have sex, and don't fool yourself, you did have sex. Penis in vagina is sex.