Hi, Does Izzy still have her stitches in? If she does, it could be redness from the stitches, or even an allergy to the sutures. They are supposed to come out after one week, when Blossom was spayed , I brought her back a week later to remove them, and for a follow up after surgery. Does it look like there is any pus from the suture site, or fever? , as these would indicate an infection. If not, then I would probably leave it until Monday when your vet opens, and that will be a week after surgery, and have them remove the stitches. Also the bump may be normal scar tissue, because about a month after Blossoms surgery, I was rubbing her belly one day, and I felt her scar area, it was raised and seemed to be bumpy. I asked her vet, and she told me it was normal healing of the scar tissue, that it would be raised for a while, and would eventually disappear. She was spayed 4 months ago, and the scar is completely flat now, it took about 3 months. If izzy is otherwise okay, and not sick, doesn't have any pus, or fever, and the scar and surgical site isn't giving her any trouble like pain , then your probably okay to wait until Monday and let your vet remove the stitches, and at the same time he can check the scar, and assess if its healing okay. If you are worried there might be some infection, you could put some peroxide on the scar area, and some Polysporin as well. She's probably okay. Don't worry, I know easier said than done, but it's probably just normal healing. Keep us posted.