Hi. I am sorry no one has responded yet. I am at least 15 years away from my last spay, so I am not sure. It looks a bit red, but that might well just be a bit of blood around the incision rather than infection of any sort. Is it puffy at all? Does it seem to be oozing a bit of blood? I really can't tell from the pic. How is she doing otherwise - eating/drinking/using the litter box normally?
Can you send a pic to your vet and ask what they think? I would try that while you wait from some other members on this site to respond. It can't hurt to ask.
It certainly looks very bruised to me and the incision looks more open than I'd have expected Is she an adult cat or a kitten?
We've just had our two (4 and 5 month old) kittens spayed and I'd note neither of their incisions looked like that - but they're also both very young and healed fast. Theirs were flank spays, can't tell for sure but yours looks like midline? One of them came home with a bit of bruising but it cleared up by day 3.
Do you have a follow up appointment due? Our vet did post op checks at 3 and then 10 days. I'd second Feebys comment above about contacting your vet if it were me. Particularly if she's not eating/toileting properly, but tbh even if she is.
In case it's useful for comparison, here are some pics of what ours looked like. This was the little one with the bruising 24 hrs after spay:
And the other little one:
And then this was day 8:
I'm afraid I don't have any pics of in between but it was basically a steady progression from one to the other without any redness or additional bruising.