Fleas or Allergies?

My new cat (former stray), who was declared "flea-free" at an exam three weeks ago, has been itchy for the past few days. Specifically, she has been scratching (mostly her ears and neck but also doing the lick-bite thing on the rest of her body) after returning home from the vet after being spayed. She's also been sometimes shaking her paws after she does it (either of the four). I would think it's just fleas contracted from the vet, but at her exam she was given a Revolution treatment which she should still be covered under. I also can't find any fleas when I brush her or look through her fur. Pollen just hit where I live about a week ago, and I'm wondering if this could be allergies? She doesn't go outside at all, but she spends a lot of time by the french doors and in the window, which are known to not seal air 100%. I'll take her back to the vet if it's not sorted soon, but I'd like to figure it out in the meantime, as my money is tight.

Thanks!

PS: I don't know if it's a possibility, but I also have been giving her pain medication twice a day after her surgery (buprenorphine). Could this cause itchiness?

You rarely find fleas on a cat, what you usually find is flea dirt.  Comb her on a big piece of white paper and if you get any tiny specks of black out, wet them with a drop or two of water.  If they colour the water red that's flea dirt.

You also need to use what's called a fine tooth comb in the UK - a metal comb with very fine teeth that will get flea dirt and possibly fleas out.

Yeah first I would come for flee dust and then if you don't find anything, talk to your vet about potential allergies and remedies for them.