You said it had white material down inside. Wonder if it is the solid pus that we see in an abscess? I had someone look at the picture who sees a lot of tumors, and he said it looked too round and smooth to be a tumor, but maybe a cyst. He thought it might be a single bot or warble fly bite with maggots inside, or an ingrown feather cyst. If you see white material it might be worthwhile to try to open it a little and try to express it. But don’t go too far if there is a lot of blood. I don’t know how late it is where you are, so you may want to wait until morning.