Thigh injection that goes past the skin layer all the way down to your muscles, where most of the biggest blood vessels are. This is often used for the intravascular delivery of AAS. Your muscles are made up of fibers, so when a needle punctures the muscle, it breaks muscle fibers and creates muscle damage, which can cause pain during the injection. After the injection, your body releases a response that causes post-injection inflammation because of these broken muscle fibers. While your muscle fibers repair themselves, you can experience post-injection inflammation that can trigger pain receptors.
On the other hand, BA/BB% concentration can also cause PIP. I would use warm cloth to reduce site pain, and take Tylenol or Ibuprofen to ease the pain.
I would avoid quads or thigh injections if possible.
Focus on rear Deltoids or Gluteus Medius or Gluteus Maximus
That can ease the PIP after the administering the solution,