Had unprotected sex 3 weeks after my period and now i'm cramping. plus he pull out. what can this be?

Several things: Of course it could be pregnancy. Any time you have unprotected intercourse pregnancy is a possibility. Your cramping could be your period getting ready to start, a pelvic infection, a bladder infection, or a colon problem like IBS to name just a few possible causes. If the cramping continues or you develop other symptoms you need to be seen. You can do a pregnancy test to help rule out pregnancy.