What are the chances of me being pregnant?

Last month me and my boyfriend screwed around a few times. From what I can remember he only fingered me once but it was for a short time and I'm almost positive he didn't have any sperm or pre*** on his fingers because his hands weren't anywhere near his penis. Another time though, we were screwing around, but I'm pretty sure I wasn't anywhere near his c** and I was on my period and had a tampon inside me. I made sure we washed our hands after we were done. We've never had sexual intercourse. I had my period on the 22 or 23 last month and according to my period calendar app, I am about 3 days late in my cycle. What are the chances of me being pregnant? Should I start to worry? Why could it be late? The only other thing I could think of that would affect my cycle is I recently lost a few pounds and I might have an undiagnosed eating disorder. I'm 5'9 and I weigh 124 pounds. Can someone tell me what's going on? Sorry to bother all y'all. Thank you for the help though and merry Christmas!