Hi Lisa
If it's real (the low platelets), it would be because she's either not producing them or her body is destroying them. Neither of these points to anything good, as you can imagine. However I wouldn't start any hand-wringing until you get the lab results. If it's abnormal, they can ask the lab to have a pathologist (board-certified vet with specialty in reading cells on slides) review her blood. They also should (if they haven't already) send out a full coagulation profile to determine if there are any other issues with clotting.
Uncommonly, the blood draw technique will cause a false reading of low platelets; in an asymptomatic cat, it should always be repeated (fresh venipuncture, clean stick with no poking around to find the vein to get new sample) and then sent to the lab.
I'm saying to wait to worry in large part because a cat with significantly low platelets (the official name is thrombocytopenia) usually has other signs. It's really a very serious condition and an affected cat is in (depending on the severity) sometimes imminent danger of bleeding to death. [Don't tell your friends all this, BTW, let's wait till they get a definitive answer.] Signs include petichiae (as in petichial hemorrhaging, like they say in CSI), extensive bruising, swelling (due to bleeding under the skin), bloody nose, bloody drool ... you get the idea.
So, the primary suspects would be infectious disease (usually viral, like FeLV), cancer, another source of severe inflammation, or (less common) auto-immune disease. This is assuming the cat is not on any new drugs and has not eaten anything toxic like certain human meds. My vote right now would be for some kind of lab error or venipuncture snafu ... Keep us posted.
PS When I worked in critical care we had a severely thrombocytopenic Dachshund in on emergency once who had a huge bruise from being carried with a hand under the chest -- it was like a grotesque smudgy handprint. No pressure on me to get the IV in with one clean stick, of course! It was nerve-wracking, Dachshunds have those difficult stumpy legs and he would have bled forever if I stuck and didn't get the IV on one try which thank goodness I did.
