My kitten's abdomen is slumping.

I am new to the group and to cat ownership. I adopted 2 kittens back
in the summer, one male and one female. Their names are Donny and
Marie. They are now 7 months and have been a great joy to me.

I want to do the right thing so at four months, I had them both fixed.
Donny is fine but Marie's abdomen is slumping, for lack of a better
term. This occurred right after the surgery but I didn't think
anything of it at the time. Her belly has sagged and it is clearly
visible. You can feel two bulges or masses in the dropped area but she
is not tender at all. Can other cat lovers who have had this surgery
please tell me whether or not they have had a similar experience and if
I should worry or not? Thanks in advance.