Sometime around June, I gave my cat a B12 injection. She had been having stomach/digestion problems since at least January (clearly in pain after eating, intermittent diarrhea, avoiding food, barely moving...) and I wanted to make sure that an inability to absorb B12 in her irritated intestines wasn't preventing her from healing. Maybe it was a coincidence, but she seemed to get over her issues pretty quickly after that, and a couple weeks later she was knocking stuff off my desk and chasing it around and begging me for food when she finished eating. You know, normal healthy cat stuff.
This morning when I was petting her, I noticed a small lump between her shoulder blades, right about where I injected the B12. It's almost a centimeter long and skinny, maybe a millimeter or millimeter and a half at the thickest. I don't think it's infected, because she didn't seem to have any problem with me touching it. I didn't actually get a good look at it, though, because I could only find it when she was curled up on my stomach in bed and when I moved to find some light she wouldn't get in a position where I could see it. (It basically disappears between her shoulder blades in any position besides curled up in a ball.)
Has anyone else seen something like this from B12 injections? Should I be worried? I've heard of injection site cancers from vaccines, but not B12. I'm hoping it's just scar tissue because I wasn't very dexterous with the needle (I had to jab her twice), but now I'm wary about giving her any more.