What Causes Menstrual Cramps With No Menstrual Period?

Hello. I usually get my period the 23rd-25th of a month. It has been coming on regularly for years. I recently had unprotected sex on July 18th & the day after I experienced menstrual cramps. I thought my period was coming on, but it was just the cramps. The cramps lasted for about 4-5 days. My period did not come on at all this week. I didn't cramp or anything. I took 2 pregnancy tests and they came back negative. I'm not sure if I took them to early or what. I'm not sure what's going on?