I grew my hair out for two years. When I cut it, I had noticeably less hair.

I am a 21 year old male. I grew my hair out for two years. 2 months ago, I buzzed my head. I noticed I hd significantly less hair. While I had long hair, I would pull my hair out from time to time due to knots etc. It’s been two months, and my hair is longer, but the total quantity of hair is still less than pre-long hair. Any possible causes/how can I help my hair grow back? No noteworthy baldness in the family or anything.