Traction alopecia is a form of alopecia, or gradual hair loss, caused primarily by pulling force being applied to the hair. This commonly results from wearing hair in a particularly tight ponytail, pigtails, or braids.
It's also seen occasionally in long-haired people who use barrettes to keep hair out of their faces. Traction alopecia is recession of the hairline due to chronic traction, or hair pulling, and is characterized by a fringe along the marginal hairline.
It's possible that what you're experiencing could be traction alopecia? You might want to stop pulling your hair up in the same way for awhile to see if the hair thickens back up. Good luck to you.