I can think of two possibilities here: either you just got a little chafed during sex or you have bred your very own Urinary Tract Infection.
Relax, it won't kill you - if you get to your gynecologist and get treated. One of the most common ways of getting one is when a little bit of bacteria gets up your urethra during sex. That's why in future you should always pee before and after sex, and if you don't already, always wipe from front to back.
If you've had a UTI before, and this isn't it, then it's likely you just got a little chafed during sex and the urine is irritating it a bit. You should heal fairly quickly down there (a couple of days).
If you have never had a UTI before (also called a bladder infection), you need to get to your doctor and make sure you don't have one now. They only get worse if untreated, and your doctor can give you a pill to make it all better very quickly.
In the meantime, drink *lots* of water. That's good for you no matter what.
~lemming, Scarleteen Advocate
"Years ago, I was an angry young man/I'd pretend that I was a billboard/Standing tall by the side of the road/I fell in love with the beautiful highway..."-Talking Heads, "(Nothing but) Flowers"