[header] How to diagnose and treat feline injection site sarcoma [title] Watch for a growing lump. [step] A small lump is normal when a cat receives an injection. However, if it grows bigger over time, that is cause for concern. 
Take your cat to the vet if you notice the lump is getting larger, especially if it continues to grow 1 month after the injection. [substeps] The lump will likely be fairly hard and not move around that easily.