Belly shot bad reaction

I (M60) had been alternating between my abdomen and thighs. I had a very and I mean very bad reaction to a shot in my belly fat. It turned into a huge knot and bruised badly. It’s been 3 weeks and the knot is smaller but still there?

I was told that I never should have been using my belly (I had never had a problem before) and just my thighs.

Anyone else have a similar reaction? Honestly, it was so severe I’ve stopped taking my treatment.