I had unprotected sex five days ago and he ejaculated in me. my period is about 3 weeks late so i don't know if i was ovulating. could i be pregnant?

Yes: If you are late for your cycle, a pregnancy test is indicated. If pregnancy is not desired, see a physician for effective contraception options.

Possibly: If your period is 3 weeks late and this unprotected encounter occurred 5 days ago, it is possible you are pregnant from an earlier sexual encounter. Take a home pregnancy test-- if negative, give it another week or two and retest if still no period. If you are not having your period, aren't on birth control, and aren't pregnant-- you should be seen.