You could be seeing a small amount of swelling as the kitty's body is reacting to the suture material or it could be a slight seroma (fluid filled sac). Most often we see seromas form after surgery when either the cat is licking at the incision site excessively or the cat is too active running around the house, up and down stairs, etc.
You are most correct, let you veterinarian check it out today and he/she may recommend starting your girl on some antibiotics. If she is overly active, you are going to have to find a way to keep her calm and quiet for about a week. If she is licking at the site, she may need an Elizabethan collar.
Good luck to you!
Just adding my dime:
1- My girls all had problems with loose poo caused by antibiotics after the surgery
The kids were recently spayed and are having this problem but I cant tell its that the cause,since they had a few worms previously - and received a anti worms again 2 or 3 days ago without no visible difference
2- If you had to avoid her licking the place and could not use the e-collar cos the cat refused completely and/or you feel pity, try putting clothes as the one you can make yourself with a tshirt sleeve (long one) its simple,works fine and bother less the cat.Just make holes for the paws, fix with plastic tape and be sure to leave the space for her biological needs,You have to work with the animal ON the clothe or wont fit ok.
I did this for mine,after buying and changing the collar 3 times ( wrong size) and trying a nylon socks clothe ( she would destroy it the same way the one that pos cirurgic one she came back with.