Josh
Relax (easy to say. I know) if you start taking finasteride today it won't kick in for probably 3 months anyway.
You're not going to loose it all in two weeks, unless you've got Telogen Effluvium, which I'll bet you have'nt
Your more than likely just having a shed.
Get down to Boots and buy Nizoral ( in a white, pink bottle approx £5) and use it 2 times a week (no more) in the meantime.
Good luck