Pregnant?! Let me tell you my whole situation..

So I had unprotected "sex" with my boyfriend exactly three weeks ago. It was the day my period ended. But..he was only inside me for about 15 seconds and only precummed. He had not ejaculated from masturbation at all that week, and promised me he did not ejaculate inside me. Still, I took the morning after pill. About three days after taking it, I had two days of light brownish colored bleeding. After some research I found out that could be a side affect of the pill, so I stopped worrying. Additionally, I have irregular period which come about every two weeks. The reasons I am now worried are because I am three days late for my period. I know this could also be a side affect of the morning after pill, but it's still concerning. Also my breasts are tender, but I'm not sure if that is because my period could be arriving, or the pill, or pregnancy. Due to my irregular periods, I don't know when to take an at home pregnancy test. When should I take a test?! What do you think my chances of pregnancy are?!