Hi zelack, are you Type 1 or Type 2?
Insulin is a major fat storage hormone -- with too little of it we cannot store fat... with too much of it we cannot burn fat, so:
If you are a Type 1 with little to no insulin, you would have been using up -- and not replacing -- your fat stores... once that is corrected with injected (exogenous) insulin your body should fairly quickly return to your "normal" weight.
If you are Type 2 with higher than normal levels of insulin but the body is not using it effectively (Insulin Resistance -- IR), "covering" food with a "top-up" of exogenous insulin to overcome the IR can lead to excess fat storage... which in turn can lead to increased IR and so on...
The trick is to balance just the right amount of insulin to manage a healthy BG level while minimising how much you take to so as to not cause excess fat storage. Arguably the most effective way to minimise your insulin use while maintaining good BG control is to: minimise the foods that raise your BG levels.