Injecting - Bad reaction from injecting heroin

[G_nome]

A while ago SWIM IV'd some heroin the way he would usually do it. Almost immediately after injecting, his hand and forearm swelled up, went red and there was wee lumps all over his hand and it felt all tingly. It looked like his had almost doubled in size. He says it was very uncomfortable but the heroin obviously dulled any pain this may have caused.

SWIM stupidly never went to the hospital to get it checked out as, at the time, he didn't care much about himself or what harm he could be doing to himself. And his hand and forearm stayed the same for the rest of the day, slowly going back to normal through the course of the day.

So, SWIM was just wondering what caused this. Too much citric? A cut in the heroin? Though no one else got these effects from the same stuff. Pushing down on the plunger too fast? Missing? SWIM really doesn't know.
So if anyone could help SWIM with this it would be greatly appreciated.