Pregnancy Scare

I (F16) and my bf (M16) are going through a pregnancy scare, I was supposed to get my period on the 17th of January however my period never came and it is January 20th. I took two pregnancy tests last night on January 19th and they came back negative. I have also been very stressed this past week and I just recently started working out. Please help! My periods aren’t normally this late and yes we have been having unprotected sex. Flo is no help as it just keeps guessing and guessing when my period will be.