Pruni - I used to have heavy periods and still have the odd one like this. I can get through a Super Plus Tampon and huge towel in about 15 minutes and I mean soaked through. For me it usually lasts about a couple of hours and I find that if I get up and about, rather than sitting that whilst the bleeding is very heavy, it seems to be over with sooner IYSWIM.
Hope it's just a one-off and is over soon.