I've had the opposite problem. My normal cycle is 21 days, with 7 days of blood loss. I was on the injection for a couple of years and bled solidly EVERY DAY for 10 months. The GP was very unsympathetic and said "well it's the Depo - what do you expect" as continual bleeding is rare but possible.
Eventually got to see a gynae doc and had an op to sort it out, but I have NEVER been the same since and still have major problems. If I had not had a period for seven years I would be skipping down the street, but I guess you've forgotten how horrible it can be by now Marymoo! 
It is usually only the first day or so, so you'll be fine soon!
Those men don't know they're born do they?
Just for the record, my next one is due the day of my marathon and the last one arrived the day of GNR! What is it about running women timing their cycles to race day?
Edited: 18/10/2008 at 08:38