To be honest it could be anythging although it looks like a cyst that Kocka had on her side and I also had one on mmy back.
Word of warning if it is a cyst.............they tend to try antibiotics if its infected and then will say just leave it...........also happened with mine. Sometimes they come up and then go down for no reason and then reappear.
However another thing they can do is lance them, dont let them do this as its much better to have then removed completely under GA, especially if your cat is young.
Kocka was about 14 or so when hers appeared and they didnt want to touch it. But when she was ill and around 18 it suddenly burst and I didnt know and the flies got to it and laid eggs.........only just caught it in time and then she needed an emergency op to get rid of it, which had it been removed when she was younger it would have been much better for her.
Cysts are like giant peas in a skin beneath the surface and if taken out in one go without bursting, this reduces the chance of infection and is a nice clean operation. Also reduces the chance of them returning.
To stop her having to wear a collar or anything, they stapled her wound which was quite big and she had no problem with the staples at all. I was really woried about them being taken out but she just lay there and they came out really easy with no fuss at all, they use a special tool. The scar healed very quickly even for a very sick elderly cat.
Good luck at the vets