What Are The Chances Of Pregnancy Through Pre-ejaculatory Fluid?

Hi there. I ve recently had unprotected sex with my boyfriend. He did not ejaculate inside me and I had a light period. I only started taking the pill that day. I understand that the pill does not prevent pregnancy for atleast 15 days, which is why I m disappointed in myself for having sex without a condom. This happened 3 1/2 weeks ago, I m due for my period this weekend. I have recently been feeling sick and I m unsure of whether it s because I m anxious and stressed. Feeling sick only seems to come when I m thinking about it. I wonder if I ve talked myself into thinking I m pregnant. I know there is always a possibility, but from my research the chances are low because there was no ejaculation, and I was on my period. However could you tell me the chances so I can have some piece of mind. I m worried that if I continue stressing, it will delay my period which will cause me to freak out more!