The anxiety is driving me mad, and I was hoping that someone would be able/willing to give me advice regarding my situation - hopefully someone who has much more knowledge regarding conception and ovulation than I do. I'm unsure who fathered my child (one of two guys), but maybe the details of the situation would be enough to help tip the odds in favor of one or the other. So here goes:

I had unprotected sex with the first guy on August 26. It's probably important to mention that he has had a vasectomy, but I know nothing about this procedure and its effectiveness. Three days later, I had unprotected sex with the second guy; he has NOT had a vasectomy.

When I went for an ultrasound, it dated my pregnancy at exactly 7 weeks; countaing back those weeks, I found that the day of my conception (if that's what it means to count back the weeks of an ultrasound scan; not sure if it's ovulation, fertilization, etc.) falls 4-5 days after I slept with guy #2. 

I can't calculate my conception date based on anything else because my periods are highly irregular. So, naturally, I'm freaking out. Would someone be able to give me their 2 cents and perhaps explain the processes a little? It would be greatly appreciated and perhaps help abate some of my anxiety.