Confused!

I'm trying to get my head around being pregnant because nothing is adding up. I came off the contraceptive pill to give my body a break and we decided we would use condoms instead. My cycle was never like clock work but was close enough give or take a couple of days. My last cycle said I should be ovulating around the 10th and on the 18th my partner and I had a little mishap, but reading up the likelihood of me getting pregnant is slim to none so whats happened? We are excited but it just doesn't make sense anyone had the same experience?