Mother/Daughter fighting since spaying

Hi,

I have a ten month old cat (Stardust) and her three month old daughter (Echo).  Yesterday Stardust was spayed.  When she came home, she has been very happy, friendly, and affectionate.  I kept them seperated for a while per the vet's instructions, but now they are mingling again.

Echo has had a very difficult time.

She's growling at her mother, hissing, and when they get near she's hissing.  The mother even gave Echo a little swap when she got a little too close for comfort.

Why is this happening?  Has anyone else ever encoutered anything like this before?  I don't understand what's happening.  Does anybody have any advice?

Stardust will smell different after being at the vets, and even more so because the spay will have altered her hormone levels. The best thing to do is separate them, and carry out introductions as though they'd never met before. Feliway diffusers might help calm the stress a little too.
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Thanks.  I figured the smell might have something to do with it.  I know the hormone level is different, but I just didn't think it would be so radical a change especially in so short a time. 

The primary smell issue will be the 'vet smell' - the hormonal changes alone would be unlikey to create this strong of a reaction.

Do the seperation/reintroduction strategies still work to get rid of the vet smell? I seperated them about ten minutes ago and I sprinkled some catnip onto a blanket in Stardust's room so I can get her to use the blanket and bring the blanket out with her scent to help Echo become adjusted.

The reintroduction is letting them get to know each other all over again, so smelling different really isn't an issue - their current scents is what the cats learn to accept. Yes, over time the vet smell will go, but it will be a gradual process, so the scent change won't cause an issue.