The cause of pain in the stomach after sex is actually lower abdominal pain
. The causes may be UTI like Cystitis
or PID - both needs investigations and treatment - like USG abdomen, complete urine analysis including culture and sensitivity and vaginal smear study.
Before doing all these, have your abdomen examined for any other local causes other than urinary and gynecological causes for the pain.
Today is 27-07-13 - too late to comment on your pregnancy