With other animals and people, hot compresses for awhile to see if there's any change. But in a rabbit nest, they are heating each other themselves.
If it's fluid, you could use a small gauge needle and suction out a bit, if there's no blood, you may be able to pull more. I wouldn't cut into it, just because the kit still lives in a nest where they pee on each other and walk on top of everyone, that will get infected fast. Even if you put on a bandaid, mom will rip it off.
If you do use a needle, use a new one for each skin prick.
It kind of looks like a bug bite to me. But thinking of where it's located, I would watch it very closely, if it gets any larger or you see loss of movement in the back end, I would start trying to alleviate the pressure and hoping the infection/swelling isn't going into the spine.