How can I prevent a pregnancy if...?

The morning after pill is very effective. I highly doubt you'll be expecting. I've taken it 3 times and never got pregnant when I had taken it. I wouldn't try any herbal remedies, most of them are fakes and will just make you sick. You took the morning after pill 4 hours after sex and the sooner you took it the more effective it is (that's sooner then I ever took it). Try not to stress because stress can delay your period. Good luck, you should be just fine.