It is actually birth control just in a higher dose, it is not exactly the same as Plan B but it will work as long as you do it early enough, so take it if you haven't already b/c I think you need to take it within 24 hours. The high dose of progesterone will cause an inhospitable environment and you may have a "period" in a few days.
And, my doctor okayed us to have sex after 4 weeks too, so don't listen to all those people chiding you about it...as long as it doesn't hurt you're fine! I also think it's great you actually want to have sex! Just maybe next time, use a condom. If you don't want to be pregnant already, use a condom, did I mention that we used condoms?!!!! ;)
Oh and my husband is a pharmacist so I feel pretty accurate about my advice! ;) Hang in there, I'm sure it will be okay!