I've had sex frequently w/ the same person unprotected. but just this last time, after we were done i felt nauseous, very tired, queezy, and gross. y?

Maybe: it's time to get on birth control and use condoms PLUS spermicidal foam, jelly, sponges or vaginal suppository. You are obviously not interested in protecting yourself from pregnancy or STD. I suggest a comprehensive GYN and large doses of sexual responsibility.

Unprotected sex: Hard to say why you felt ill shortly after sex, but unprotected sex can lead to pregnancy, so if your menses is late, a pregnancy test is in order.

Hard to say, but...: ...it only takes one time to get pregnant. Consider a home pregnancy test at this point.