when i first started shootin my glutes like a day after my lower back would be a little sore, I have no problems now. I mainly inject in my quads though..but has this always happened, is this you first time stickin your glutes?
I don't believe the actual injection could cause back pain. Perhaps the torque you put on your back when you reach back for the injection tweaks something. Try lying on your stomach on a bed and using your free arm to leverage a twist in your upper torso. That way you aren't actually contracting your lower back and obliques.
its happen to me, i think sometimes the oil is putting pressure on a nerv. its sort of like an un realaxed feeling your back.
maybe im injecting in the wrong spot. where exactly in your ass is it suppost to go. i do it like 2-3 inches lower than the very top of my crack and off to the side from the very middle of the one cheek.