PLEASE HELP...???

If I was on my last day of ovulation and had unprotected sex and didn't take a plan B until 36 hours after sex does that mean I can be pregnant if so when can I take a test too see??? 12:54am on September 14 (had sex) September 15 at 12:00pm (took the pill) what are my chances I have a lot of cramping on my right side and lower back pain my test keep saying negative tho???