Have you tried straining out any contaminants? Could the acidity level be causing issues? Or perhaps you're injecting too much at once? Honestly though, I'm clueless since I've never felt that burning sensation myself and I always switch up my sources. Update: Turns out others mentioned the same thing during the initial stages.
Bought reta from two different places that both have solid reputations and reviews. One version stings during the shot and the other does not. Lost forty pounds since mid August so both versions work, but the injection feel is very different. Used the exact same bacteriostatic water for both so that is not the cause. Any ideas would help.