What Causes Irregular Periods?

Hello, I am 21 years old and I have been sexually active since january 9, 2015. I usually used to have regular periods, ranging in a cycle of 28-31 days. My period on february was very late though and I even took a pregnancy test two times and it was negative. After a 38 day cycle my period came and it seem to have a normal blood flow and lasted the same as my regular periods. However this month it is late again. All of the times I have had sex has been using a condom and my boyfriend would also pull out before ejaculating in adition to the condom. All of the times we had checked to see if the condom was broken and the condom was fine. I was supposed to have my period on around the 9th or 10th of march, however I seem to be late again. I took a home pregnancy test on the 12th and it gave negative results. If my period does not come on seven days I will probably take another pregnancy exam. But I was wondering if there could be any other reason for my period to be so irregular ever since I started to have sex?