Pregnant? 😬😓😥

Ok so on eve I was suppose to start my period this past Wednesday on the 29th, but I've been late for three days now. I'm irregular on my periods and I'm not on the pill. I've been stressing alot due to school and also because I had anal sex on the 24th of February and he did cum inside of me just a little, is it likely that I can get pregnant or I am pregnant or is it just because I'm stressing so much about school and that my period hasn't came yet? What can I do to stop stressing about it? Please help I'm so worried😞