Scarred tissue irritation...
If you only shot glutes you are gonna havea problem within 5 years. I know I did.
Bascially you are injecting forming a depot that takes a 1-2 weeks to deplete and reshoting into the same depot.
The irritation from the bascially violating the depot will get you a hand full of scarred tissue.
Only solution is find other injection spots.
Hell I done traps and they hurt like a mofo.