This is a great post and right on the money. The site probally was not infected, just irritated. This happens to me when I shoot my quads also, but they are huge and vascular so I should have known lol
I wish I could shoot em cause its a great big muscle that would be a good place to put gear.
I used to shoot my quads a lot, but I got fed up with the never knowing if I was going to have that irritating pain post injection...it's my understanding that this comes from all the nerves in the quad...I guess they are irritated by the pin, I'm not sure exactly but that seems to be the case. In short, for myself, it's a 50/50 shot, so I don't mess with it any more.
...and you're right, it's a huge muscle to inject into, and very easy, but as you well know, all of us huge guys have tons of other huge muscles to inject.