Could be the prep pads but I'm not a mind reader or a psychic so I am only going by what I know
I am a nurse, so I give injections regularly and the quads are one of the areas we need to be more careful due to the abundance of blood vessels and nerves. The whole "fast in" technique that people are taught for self quad injections is actually dangerous because you can sever a nerve. I know because it happened to me and part of my leg is permanently numb now on the skin surface.
Even though it's more painful, slow and steady is better for the quads! Deltoids who cares.... just nail it.