nauseous after sex. why?

No idea. Was it your first time? Then perhaps you are nervous? It is not a pregnancy sign a few days after sex.

I hope you are being safe and protecting your health and future during sex.

yes, it was my first time. My mom suggested that my cycle just may be coming soon. & me & my partner did not finish...

If you do not use the pill or condoms, there is a good chance that you will get pregnant sooner or later. Not finishing is not a form of birth control. So please take care and be careful.