One problem is that you are outside of the character describing what is happening. I have been rather rough on my characters (shot, stabbed, tortured, etc) and I try to crawl inside that character for a moment and be him/her.
Pain is not felt immediately. It can also build in intensity.
Ask yourself who is Johnny? How will he respond to the stimulus you have chosen?
How intense is the pain? What duration?
Imagine that you are Johnny - feeling the sudden sharp pain, not quite understanding it.
Since he is standing when he receives the injection, I don’t think it is an epinephrine shot.
Like you, I prefer third person narrators. In my piece, my secondary protagonist takes a bullet for her mentor and I have her feeling as though a tsunami just hit her. Her pain comes in waves, exacerbated by any movement.
The last time you were injured, how would you describe that pain?
Johnny staggered back, the sharp blow to his chest more devastating
than he thought. It hurt to breathe now, and his 6’1 frame had
crumpled under this sudden blow. His hand strayed to his chest, clutching at the syringe. He could feel no blood - thought he’d been shot. He hated needles, always would. His
breathing sharp, ragged and shallow, he glanced at his assailant, who
was counting. Seven, six - what? Three, two the world faded and he
fell, striking his head on the pavement.