Help I need some serious advice!!!

Ok so this is actually crazy. I slept with a guy the other night that I knew from high school. I hadn't seen him for 2 years and we got caught up in the moment. We couldn't find the condom at the end of it but I figured it fell off the bed or something. Well....fast forward to the next day, I found the condom.....inside me. 17 hours later. 17 freaking hours. I had ended my period 2 days before, so I know my fertile window doesn't start for a few more days. I think it's too late to take the morning after pill (slept together Saturday night, it's now Tuesday). I'm also from the USA but living in Australia so I'm unfamiliar with their policies on contraception. My question is how worried should I be about pregnancy? I'm not too worried about STDs (both of us were recently tested) but I do plan to be tested again within the next month. The end of the condom was outside of me, so I'm not sure how much sperm could have made it in. I also just ended my birth control pill which I had been on for 3 years last month. So yeah I'm freaking out!!!! 17 hours!!! How didn't I notice? :( I feel so stupid.