The lumps are probably a normal consequence of the surgeries.
Spaying a cat or dog involves surgically removing the uterus and ovaries. These organs are located inside the abdomen.
To access the abdomen, the surgeon must first cut through the skin on the animal’s underside. Beneath the skin is a layer of fatty tissue, which is cut through next. Finally, the surgeon makes an incision in the muscle that lies beneath the fatty tissue. At that point he or she is able to operate inside the abdominal cavity.
When the surgery is complete, the surgeon must close the openings in the three layers mentioned above (muscle, fatty tissue, and skin) with stitches. The most important layer is the muscle layer. If it comes open for any reason serious complications can occur.
Therefore most surgeons are very aggressive about suturing (stitching) the muscle layer. This may lead to bunching up of the muscle. The bunching is not painful and does not hurt the animal in any way, but it can cause a lump to develop underneath the incision.
I suspect that the lumps you have noticed are the result of normal postoperative tissue bunching. But there is only one way to be sure. I’d recommend that you have a vet take a look at the areas to confirm that nothing is wrong.
About the photo: Lillian shows off her spay incision.