What was that?

The other night while getting Ellie ready for bed, she let out a scream like I've never heard before. (This is Brian, by the way)
I've heard her progression of cries. It starts as a whimper (sometimes she enunciates the 'w' in 'whaaaaa.' It's hilarious.) Then she goes into a full cry, and eventually a scream that tears at your heart because you can tell she really is hungry or upset and can't relieve it any other way.
Well, as I was changing her into her PJs, she skipped all the preliminary cries and went straight for the deep scream, louder than I'd heard in a while.
I stopped cold and looked around her. Had I pinched her slightly with a snap? Had I pulled one of her fingers back as I pulled her hand through the sleeve? What had I done to cause this? It was clear that she was hurt because of the intensity of the scream.
It was then that I noticed Ellie had inflicted this wound herself. As the scream got louder, she pulled harder on her own hair. And she it hurt more, she clinched and pulled some more. It took a second or two to get her hand to loosen. The look on her poor face was one of pain and utter disbelief that this could happen.
It's crazy how bad I wanted that scream to stop. Not for my own ears' sake, but because I didn't want to see her hurt and upset.
I guess she's not as thrilled as Dorothy and me that she was born with so much hair.