
Maybe she has an infection going on? Its good you are taking her for a check up.
Our vet generally wont spey, as you say, he likes to do them inbetween seasons, but when my bitch had her pups in September, she was bleeding heavily and when the pups were 10 days old, she had to have an emergency spey because she was getting very anaemic and the iron tablets were not working enough to stop the bleeding. It turned out that she had a ruptured artery in her uterine horn.
So even though her uterus was obviously still enlarged and her hormone level was high from having pups, he speyed her as we didnt really have a choice. So surgery is possible if you are worried, its just slightly more risky with an enlarged uterus.
Maybe they will scan her again for you to see if anything has changed inside. Hope you get on ok though x