Reaction to anti-inflammatory injection?

I noticed a lump on my cats back about a day or two after she was given an anti-inflammatory shot. It definitely wasn't there before and its about the size of small grape. It's not painful, but I'm worried because it's still there after two weeks. I was already planning on taking her back to the vet in a month like the doctor recommended, but now I'm wondering if I should take her back sooner. The problem is that I've already spent 450 dollars taking her to a clinic and an emergency hospital. I only want to take her if it is urgent. :/  

Does anyone have any experience with something like this? Should I be worried about this?

Update: The lump is gone. I'm guessing it was a reaction.   

I would call the vet's office and ask them if this is a normal reaction.  Sometimes it's normal to have a knot at the sight of an injection until the med has been absorbed into the system.  I'd watch it and if it doesn't go down, I'd get her back to the vet.