Burns 'down there' especially when peeing?

For the past week about, it's been burning a lot down there. And it hurts like hell to pee. I have flank pain and bladder cramps. I've been on Bactrim for a couple days, but it isn't doing anything. And I've been drinking a lot. (not alcohol). What should I do? I'm allergic to cirpo and doxycylcin.