The knot in the middle of the stomach sounds suspiciously like the infamous canine umbilicus, or belly button. I discussed canine and feline belly buttons in a recent post.
However, the mass adjacent to the nipple is more troubling. It may a mammary tumor. Malignant mammary tumors are known as breast cancer.
Female dogs and cats have very low rates of breast cancer if they are spayed before their first heat cycle. However, older, unspayed females develop mammary tumors at relatively high rates.
Most mammary tumors in cats are malignant. In dogs, many mammary tumors are less aggressive. But no matter what, the mass needs to be checked by your vet.
You say that your pet will be going to the vet to be spayed soon. The veterinarian should assess both areas before surgery. He or she will probably want to remove any suspicious masses at the time of the spay surgery. If a mass is removed, a laboratory pathologist will be able to analyze it to determine if it’s dangerous.