Missed period

Kayla

My periods are very regular and come on time and last the same amount of days. But this time I am 5 days late and I’m about to miss my period overall. I am sexually active with my boyfriend of 2 1/2 years and not once have we never used a condom, we are extremely safe. So we picked up a pregnancy test and it came back negative. Does anyone have any suggestions as to what is going on?