What Causes Delayed Periods With PMS?

I took three pregnancy tests. one every week for three weeks. They all said negative. I've had missed periods before and not been pregnant, but I had unprotected sex with my boyfriend about two months ago... I'm concerned I may be pregnant. I haven't had morning sickness and by breasts haven't grown. I had cramps last month, but no period. The crams lasted as long as my periods usually do, this has happened before as well. and I feel like I'm starting to pms this month too. Help, please.