Just as Kenny said, a simple histamine response to the dope. Years ago, before I had really gotten heavy into opiates I had a shot of Morphine at the hospital and I got hive-like welts at the IV site and up my arm. The doctor explained it's more of a side effect than a true allergy. So no worries mate. Now, if your whole body develops hives and you feel your throat closing up, and super intense itching then that's cause for alarm - that would be an allergy. Almost every time I shoot dope I get an identical reaction as your girl. Now, I usually take a benadryl before I bang which not only reduces the welt at the site but also adds a little nod and any generalized itching associated with any good dope buzz. Finally, the reaction is most common with morphine and semi-synthetic like diacetylmorphine (heroin). Very few people develop this with fully synthetic opioids (i.e., dilaudid, oxycodone, etc.). Hope that helps!
