I had my period in 7/27 4days later mi and my bf had sex. 2weeks later I have been having a lot of lower cramps in my belly and feeling very tired?

Hope you feel better. Hope you feel better by now. If not, a visit with an ob-gyn will be able to address your complaint. Pregnancy test will be able to address your concern.
Symptoms. Your symptoms may or may not be gynecologically related. If you do miss your period - would go ahead and take a home pregnancy test.