To the OP -
Try not to worrry - easier said that done, I know! I was in your situation about 2 years ago, but I took the morning after pill even later (about 60 hours). I was fine, although my period was a week late.
Well done for thinking quickly and doing the right thing by taking the pill. There is only a tiny chance that you will be pregnant - as your doctor said, if you had sex right at the end of your cycle, then you didn't have sex in your "fertile" time, which is in the middle, when ovulation occurs. Try not to stress too much - you will possibly delay your period just by worrying about it (as I did). If you period IS a couple of days late, take a pregnancy test - you can get one for about £5.
Just a final note to say that if you are pregnant (very unlikely, but still) - think carefully before opting for the chemical abortion. It might just seem like a problem to you, but you would in fact have a tiny child inside you; a bit of life who would grow up into a person. Think carefully and remember that its not just a thing, but a person, just like you. I was 17 when I thought I could be pregnant, and we made the decision that we would keep the baby if I was, just because I couldn't handle the idea of aborting a life.
Keep positive though - the chances are hugely, hugely stacked in the "not pregnant" aisle.