My periods are very regular. I had been dating a guy for over two years and did not use any birth control and never got pregnant. I am now engaged and I just found out that I am 3-4 weeks pregnant. My last period was January 18 2012. Can you please tell me when I was likely to conceive, I made a horrible mistake and slept with my ex he pulled out late so now I am so stressed about this situation I can't even sleep.