Watch these two videos.
Here is the close up on her left shoulder.
VIDEO
And here is her right pant leg.
At 0:12 second mark you can her a pop followed by metallic sound.
It could be the sound of the spring blowing out the back end of the diazepam injector. Watch how he puts his right hand on her left shoulder then begins feeling around - you can see him push like he's pushing the injector with his palm into her shoulder. At that moment is when she really begins faltering.
If he didn't line it up properly and it hit hard bone instead of soft flesh then the injector would blow the spring mechanism out the back end.
The spring is what bounced off Hillary's right pant leg. You can see it coming from behind the bollard in the slow-downed video.