I don't think it has anything to do with the spay. She is just maturing.
I have found that food aggression is the ONE thing I haven't been able to break.
I recently started a new technique...I hold her food (made extra good w/ wet food, egg, or cottage cheese). I hold the bowl in my lap (I am on the couch) and pet her head, and talk gently while eating. I have no idea if this will work, I'll let you know
Many suggest to leave your dog alone while eating/chewing on a bone. I did that for a while, but I don't like the idea of my dog having these aggression issues.