I had sex one day before my menstruation came. i'm feeling sick three weeks after. is there a possibility i could be pregnant?

See your MD: It is not uncommon to have an abnormal period once. Stress strenuous exercise, medical problem like diabetes or thyroid condition are a few reasons one might have repeated episodes of abnormal menses. Some women have irregular menses and pregnancy is possible. See your md, if appropriate do a pregnancy test.