Precum, 10 seconds, morning after pill, pregnant?

I'm 19 years old and I know this topic has been talked about before but I was wondering for my case... I usually have a normal cycle of 28-30 days but for the month of June, I think I had some kind of spotting for days then my normal period came on the 1st of July to approximately about 5/6 July. I was intimate with my bf on the 16th of July (Saturday) and he was in me for less than 10 seconds at around 2pm. I realized it hurt so I told him to stop. He came about 2-5 minutes later but none on me. I got really paranoid on sunday and decided to take the morning after pill (postinor-2) on Monday at 12noon. So it was about 46 hours after the incident? I took a pregnancy test on 20th July (wednesday) and it's negative. Doctor told me to wait for my period to come and if it doesn't, take another pregnancy test then. I just want to know how high my chances are of becoming pregnant. I know its wrong and all to have unprotected sex but I really regret it and won't do it again. Anyone please help? I'm so worried till I can't do my school assignments properly at all. :/