You need to set up a doctor's appointment ASAP. Mention that you experience painful painful urination when you make your appointment too.
You could have a urinary tract infection, so until you the doctor is able to see you, we highly recommend taking cranberry pills (you can get them at a health food store). Cranberry helps keep your urinary tract healthy, but unfortunately, cranberry juice cocktail at the store tends not to do much good.
When you get to the doctor, be honest with him/her about your sexual history. Also, it's a great opportunity for getting tested for STDs if you haven't done so already. Good luck!
"The level of our success is limited only by our imagination and no act of kindness, however small, is ever wasted"--Aesop