This is very common with quad shots. If you have been hitting quads for years, I'm surprised you didn't experience it sooner. You passed through a vein, on your way in. If was close enough to the surface that it shot out through the channel you created with the needle. I've had it happen a few times. Once, the blood shot a few feet lol. Since then, I always have a towl under my leg, as I sit on the couch and pin quads. The quad contains a dense network of veins compared to the glutes. You'd be less likely to experience it in the glute. 

Scar tissue, on the other hand, doesn't bleed. Thats part of the problem. Scar tissue impedes blood flow. This can make it harder for gear to break down and can more liekly lead to infection over repeated use. If you feel tough areas in the muscle, as you try to push your in, or hear "snapping", "popping" sounds, you have a good deal of scar tissue built up on this site. I use the foam roller and get massage to help break down any scar tissue that may be forming. Thankfull, that doesn't seem to be your problem today!