What kind of Shot was it. My Coco has lump from her Depo Shot that she got over a week ago. last time she got that Shot it took 2 Months to go away. My other Cat that died got a lump from the distemper shot that took a few weeks to gget rid of.
It would be best to call your vet just to let them know that your cat has developed this lump. At the very least they will want to note it on her chart. Most likely they will want to see you in a few weeks if they lump is still there.