It sounds like your cat has an aural hematoma--basically a big pocket of blood and fluid in the ear flap. This generally occurs due to excessive scratching at the ear or a hard blow to the ear. It is fairly common in floppy eared dogs with ear infections
. I have seen it in cats, however, generally when they have a severe ear mite infestation.
This is very painful to the cat, as you can imagine. The fluid needs to be drained off and often a minor surgery done to the ear to keep the fluid from accumulating again. You will need to take your cat to the vet and I'd recommend doing it today, rather than waiting until Monday.
Some vets will make an incision the length of the ear or pocket, drain out the fluid, then suture the ear flap through both sides along the incision--basically tacking the ear flap back down to prevent further fluid accumulation and leaving the incision open to allow further drainage. There will be some scarring, but most cat ears will stay pricked if the surgery goes well.
Another option is the insertion of a "teat canula". This is a small tube (originally used in cow teats) that will allow drainage of the ear and only requires the stitch to keep it in place. Cosmetically I think the ear can do a little better, but it isn't always an option in the upright cat ear. Works great for floppy eared dogs, however!
Some vets will have variations on these surgeries. In some cases (usually when the pocket is pretty small) I have just used a needle/syringe to remove the fluid, then injected some steroid to help with inflammation. The fluid may need to be drained several times, but the ear seems to heal pretty well.
The main thing is to find out why the little capillaries in the ear ruptured and started to bleed and to treat that problem. If you don't do anything, the fluid will eventuallly be reabsorbed, but the ear will scar down and can cover up the opening of the ear canal, which will cause more problems down the line.
Hope that helps!