a tiny bit of his cum gets in your underwear and you donr realize it like
I don't know how to explain it but.. yeah
NO, you cannot and will not get pregnant from that, in the vagina, sperm will survive for 3 days, outside of the vagina sperm die in less then an hour. There is no way for the sperm to reach endometrium (the lining of the uterus) from your underwear. You have nothing to worry about. Just as you cannot get pregnant from a sperm in bath water, a pool, on sheets, or on a towel.
Only if it gets deep into your vagina.
But the sperm will probably dry out and die before that happens.
well it takes up to 3 days for sperm to die out, so you could