Greyhoundgirl, I don't know.

Jenny may have simply had a reaction to a "bite" ( insect, ? bee?)

As far as cysts, no, I don't think a cyst, left un detected/treated, would go away on its own. But, I have no experience with cysts on dogs either.

Cancer? Again, a NO, lumps and bumps that are cancerous will grow, and , spread .

I am wondering about this "bump" on your Jenny!!!!

I am glad that you have noticed it is smaller!