It is good that you are taking her to the vets.
Another possibility is that the lump is a stitch under the skin. When they make the incision to do the spay there are several layers of things they have to cut through. There is the skin, muscle and I think there is a third layer of something, but I can't remember what it's called. Any way, the layers of different types of tissue all have to be sewn back together under the skin with the skin being the last layer.
Sometimes the body will reject the stitch and cause a little lump that could end up looking like a pimple. The body naturally tries to push the stitch out and the vet can remove it.
Sometimes the body will just build up scar tissue around the stitch, which makes a lump.
Most of the time the vets use disolving stitches so once the stitch disolves the scar tissue will fade away or flatten out and you won't notice it as time goes by.
But it is good to take her in to get it checked in case she needs some antibiotics or needs the stitch removed. Either way, it's nothing major but it should be looked at.
Sandy, Koke and Lelaska's Mom