Am i pregnant or is it an effect of my diet?

about 3 weeks ago I had sex. We used a condom. I had a normal period about a week after that. But now I am experiencing major pain in my lower abdomen and some discomfort with my bowels. About 2 weeks ago I started the cabbage soup diet and then when that week ended I ate normally (which isnt great) so i am not sure whether I am feeling early pregnancy pains or if I am just having this pain because of the extreme change in diet?