What do you mean by swelling exactly? When the skin get just very lightly swollen, a bit red and kind of bumpy, and perhaps itchy, but not really painful, that's normally a histamine reaction which is very common with opioids. If it is very painful though, that sounds like a missed injection, blown vein, or hitting a nerve or artery. Don't inject into your hands, the veins there are very delicate and easily damaged, and there are also a lot of nerves etc close to them. Would you like some injection tips?