what would cause a cats red blood cell count to rapidly drop over several hours? Serious answers only, please.?

my cat became anemic after her vaccines.

There are several different things, including several toxins, that can cause red blood cells to drop. Internal parasites, feline leukemia, kidney disease, cancer and internal trauma are also causes.

hopefully your vet can figure out what is going on quickly and help your kitty out

Connie S is right about vaccines, cancer, internal trauma, and parasites, but I figured I'd also note a few other factors:

Onions and garlic can damage their red blood cells and cause anemia in a short time, too. I'm not sure *how* fast, however, though I'm sure that depends on how much kitty had.

Tylenol and Ibuprofen, but I do have to doubt that s/he managed to get into those bottles, though if you have a child around that got into it and dropped any..

And if s/he has kidney or liver damage, those can also influence anemia (The kidneys produce a hormone (erythropoietin) that stimulates bone marrow to make red blood cells. The liver produces factors that enable blood to clot.)