What Causes A Rash On The Calf?

My son has a rash on the back of his right calf. He says it makes the muscle in his calf feel like it is pulled and it hurts. 3 days ago he had morphine and phenegren injected into an I.V.. The rash only showed up today. He says it doesn't itch only hurts real bad and gets real stiff it he don't keep using it.