Is that her umbilicus? My (male) horse developed a lipoma cluster there in his late teens that looked similar from the outside, but of course, lots of things can be possible causes. His lipomas were quite firm to the touch, because in clusters. I didn't know what it was until he rolled and it came off one day. I only found it because the donkeys were licking it and he had a small bleeding wound and I put two and two together. Classical clusters of fat globules; the vet removed a couple of stragglers from the wound edge, and he's healed up good as gold.
But, like I said , could be anything, and unlikely to be the same thing my horse had... if it's an insect / spider bite or allergic thing, it will get smaller eventually; if it doesn't, you could get it evaluated.
SueC is time travelling.