The missus cat got something like that, but it wasn't after any vaccinations as the vet thought it might've been.
Seems "Tristan Cameron the Black" lept and landed funny.
He got what amounted to a massive bruise, which did this funny puff up and after it healed it did the fatty lump like thing.
Freaked us out.
It did go away.
But a fatty lump on the spine IS a cause for concern as it may pinch nerves.
Especially if it is close to where vaccinations are given.
(Sometimes is a skin reaction to vaccinations or so I'm told.)
I don't recall what can be done about it though.
I ended up having to go back to the office tonight to get some information I THOUGHT I had with me. It was worth it though, because it saved me having to get up at 4 am to drive to Harrison before work in the morning to pick up kids. I am convinced that one of life's most useful skills is the ability to delegate. I am tired enough without losing extra sleep!