Periods late by 13 days.. missed a month

Hi. So I am in a situation and I need help! I have very little knowledge. I am a virgin. Last month i started my periods on 5th oct. After i had stopped me and my bf met up. Now we got skin to skin and he had ejaculated on my vagina. We have had no sex. I have now missed my period its been 13 days. I did a pregnancy test few days ago and it said not pregnant. However i have had cases before like this missing my period but having no contact with a guy. I missed my periods in June due to stress. Recently i have been stressed out too. Please advise :/