I've been having stomach aches/tightness and frequent urination the last time i had intercourse was about two months ago, since then i've had 3 periods...last one was a day late but still fairly normal, i took 2 cvs brand hpt's last week...both negative.

Urinary infection?: The most likely cause of your symptoms is a urinary tract infection. You need to see a doctor for a urine screening test for this. If it's negative, then your doctor may want to do a gyn exam and check you for STD's. Pregnancy is VERY unlikely.

May have infection: You may have a bladder or sexually transmitted infection. It is important to see a doctor for a urinalysis and pelvic exam. Either type of infection, left untreated could become serious. Best to get checked soon.