When could I have conceived?

I just got my BFP!! But I'm a little puzzled--I'm going in to check with the doctor on Monday, but I can't figure out when I could have ovulated! In fact, I was just about to start my first round of Clomid, but my period still hadn't kicked in, so I tested just in case and it was positive! My last period was on July 3rd, but I actually had an ultrasound and blood work (we were worried about an ectopic) on August 10th, and both were negative then...so sometime between now and then, an ovulation must have sneaked in! I've always been quite irregular, so your guess is as good as mine!