Hello and welcome...
Oh, believe me, no need to explain the meowing sound your kitty is making, we've all heard it before.
Are there other cats around outside? There is a possibility a small bit of ovary was left behind...which would cause your kitten to go into heat. This could also be normal behavior for only 1 month after surgery. I've never been around a female after being spayed, so I'm not the most knowledgeable person when it comes to this issue. Hopefully someone who knows better will see your post and be able to advise you better.
TikiBacca, Hello.
Has your kitty girl's crying issue resolved yet?
Did you get her declawed at the same time?
I think it's best that you speak directly to the vet who did the spaying. Three kitties of mine would have passed very early in their lives if I'd taken the Receptionist's advice. Take the kitty in for a recheck. Jade59's advice is sound. Just in case there is something else going on best have the vet check her over.
Blessings,
Melinda