Sounds similar to my experience - I'm 27, was losing my hair very slowly from my early 20s (so slowly I didn't notice till I looked at photos later) until it suddenly got much worse in about December last year. I started on finasteride in February (no other treatments though). Within 5 days or so my hair started shedding by the load, and didn't stop - it also got really oily. After 6 weeks the dermatologist I saw told me to stop as he thought it was alopecia areata. Once I'd stopped, within about a week it went back to (almost) normal, i.e. hair loss was same as before I took finasteride (noticeable, but much slower).
Then I saw a trichologist in May, who told me to take finasteride just in case it was male pattern baldness and also to get my scalp clear as he thought the "sebacious material" could be a factor. Same thing happened - after a few days the hair loss speeded up massively, and didn't stop. It got so bad I stopped after about a month. The trichologist wouldn't listen and insisted finasteride wasn't causing the increased hair loss - it could only be the scalp condition. Shedding never starts that quickly, he reckons.
So I start finasteride again towards the end of July, and the same thing happens yet again. Then about two weeks ago, when I requested a repeat prescription (against my will!) for the stuff, there was a mix-up at the doctor's and they lost it so I had to put in another one a few days later. As a result I ended up not taking it for about 5 days after I ran out, and before I got the new box - and yet again, the hair loss slowed down a lot and my hair looked a bit fuller, somehow (can't explain it myself). I start on finasteride again, and guess what... after two days, massive shedding starts again. Trichologist still won't listen, I reckon the makers of finasteride are paying him to promote the stuff... he's told me to give it another eight weeks before we decide whether it's worked or not. Well I'll give it 8 weeks, but not a day more...
What I want to know is... since it looks like I'm not the only one this has happened to, how do we convince the medical profession (and the drug companies) we're not just making this up? Why can't some kind of tests be done on people who say this is happening to them to find out what's going on - what are they afraid of, that we might be right? I hope things get better for you, please keep us posted as to what happens.