If the condom didn't actually come off and there was no obvious spillage you should be fine.
For future reference, provided your boyfriend is using the right size of condom, the usual reason for condom sliding up/off is staying inside for a while after he cums. Then his erection begins to subside and the condom slips up/off as he pulls out. Best way to preven this is for him to reach down and hook a finger over the rim of the condom to stop it sliding as he pulls out.
Yes, if in doubt take it. But act fast, you now need the pill called ellaOne as this works up to 5 days after sex. See a pharmacist ( you may have to pay, depending on local arrangements) or go to a contraception clinic or your GP ( both free). And sort out some other contraception for preference, you can use condoms as well!
Having a coil (IUD) fitted if the best way of preventing pregnancy after a condom accident, so ask about this as well. Good luck
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