Suggest Treatment For Delay In Periods

hi, I am very concerned. My periods are usually irregular. my last period was on December 10th but ever since I haven't gotten my period, me and my boyfriend kind of had sex and by that I mean not all the way I am scared because of the seminal fluid. is there a chance I could be pregnant? I took a pregnancy test and it came out negative but I'm still concerned. I hope you can help me.