I could see it happening with meth, either at higher doses or for someone who is naive (due to higher circulation). If it is happening with heroin, it's probably an allergic reaction, which isn't too uncommon. AFAIK (and this isn't too far), the allergic reaction from opiates usually manifests itself as itching, and can be minimized with OTC antihistamines. But I'm not overly experienced with either drug, so I could be full of it.
Have you tried in OD? Might have some more experienced people in that direction.