Newly Spayed Cat with a lump

So, I still have my foster momma from the past litter. She was spayed on May 22. Her incision has looked awesome from day 1 and she's had a great recovery. Tonight, I picked her up to have a quick look and noticed a lump under the incision area. It is kinda oval in shape and about the size of a large grape. It does not bother her for me to touch it, there is no redness, oozing or other signs of infection. She is eating, drinking and playing and appears very healthy.

I do have plans to call the vet tomorrow. I will check to see if it's bigger in the morning.

Any ideas?

Almost all the cats I've had spayed have had a lump under the incision area about a week after the surgery. I think it's the internal stitches starting to dissolve. Or scar tissue developing. At any rate, it's usually gone a couple weeks later. Do ask the vet, but I think it's normal.