Could I be pregnant...?

I lost my virginity earlier this month and on my second time, my boyfriend put the condom on the wrong way round, then realised, thought nothing of it and put it on the right way. He didn't tell me until after , and now I'm worried that some sperm could've entered me from the outside of the condom? I know it's quite unlikely but we'd had before so he could've had quite a lot on his , especially as it touched the tip. This happened three weeks ago and I have missed my period by one/two weeks. Should I be worried? Please help :) xxxx