it's a condition known as lipodystrophy
Lipodystrophy - Wikipedia, the free encyclopedia
when you inject in the same site over and over again, you get fat hypertrophy - in the same way that muscles can swell with regular exercise, so can fat cells - it's a reaction to sticking needles under the skin.
you can reduce the effects by
1) rotating injection sites - the stomach is a very large area, you can use all of it to inject into.
2) change your needles - needles are designed to be single use. Many people however use them more times than this - I change mine daily and this seems to be OK.
I after 25 years of injecting into my abdomen I have a thickened band of fat. I only notice because I'm quite slim, and reasonably muscular. Rotating sites and changing my needles regularly prevents it getting out of control.
one issue you should be aware of, is that injecting into lumpy areas of skin can cause problems with insulin absorption - you can get slowed absorption, followed by a sudden rush.
