I had unprotected sex three days ago during fertile window, am having slight cramps, headaches and slightly nauseous could i be pregnant?

Yes: You can become pregnant any time you engage in unprotected sex. You can also catch a sexually transmitted disease/infection by engaging in unprotected sex, too. Best way to protect yourself is 2-fold: 1) use some form of hormonal birth control (contraception) and 2) make him wrap it up every time. If you're worried, consider emergency contraception if you're within time frame http://goo.gl/eMC4up