Cat has lump after a week of being spayed

This lump appeared after a week of her being spayed. The sides are soft but the middle is kinda firm. Anyone know what this is or has had something similar happen to their cat?

She is eating healthy & acting normal so far and I gave her a treat to bait her into letting me touch it & it doesn’t seem like she is feeling any pain. I am going to try to take her to a vet tomorrow