It's possible that your cat is having a hypoglycemic reaction to the shot, so I wanted to get that reply off quickly (hypos are very dangerous) before adding details or asking questions. There are other possible causes for the symptoms, but hypoglycemia is an immediate emergency and I don't think that a small amount of honey/karo would do any harm with respect to other possibilities.
So, now the details and the questions (but don't stop to answer these before doing the karo/honey):
1) What was your cat's blood glucose level at the vet (I assume you aren't testing at home, but if you are, great! and please get a test ASAP!)
2) How many hours since the shot?
3) How long was your cat not eating, and is she eating now?
4) Did your vet test for ketones?
5) Did the vet give any fluids?
It's very hard to say what is going on right now, but the "cannot stand up" is very worrying. Please post back when you can and we will try to help, but if your kitty's condition worsens the emergency vet is the place to be.