Six months ago, literally seconds after an orgasm, my clitoris suddenly started ... I don't know ... spasming? It was really, really, really painful - I broke out in a cold sweat and curled up foetally on impulse. It lasted about an hour, then sort went away again, only whenever I was sat down I could feel the pain of the pressure on my lower half, and sitting for a period of time became very uncomfortable, and again I would end up in so much pain I'd break out a sweat. Since the first incident, I've become embarrassingly incontinent (I'm 19 - shouldn't be happening yet!) and have to keep a secret bucket in my room for fear of someone being in the bathroom, because there is no way I can hold it in after the moment I need to go. After urinating (usually a very small amount, usually strongly coloured) the severe pain returns, and my pelvic muscles basically 'refuse' to stop pushing. Once that's sort of 'over', I then have to put up with the dull after-pain, then a few minutes later I need to urinate again. This usually happens six or seven times in what I call my 'urination cycle'.
The pain sometimes disappears completely, however, and I can go for days totally normal (with only a little discomfort for a second or two after urinating). I've found that with masturbation, the pain fades away for a while afterwards, and if I don't masturbate for a few days or more the pain becomes unbearable.
I'm a university student, and because I've been unable to face the pain that comes with sitting down on most days (particularly on those hard seats in the History lecture halls! Ouch!) or the embarrassment of having to run out or have an accident, I've been skipping lectures.
I really need an idea of what this is - can anyone help?
N/B: I haven't had penetrative sex for two years - could this be a latent STD?