What causes hair lose?

Ces

So I was at the salon and the lady that washes my hair told me I’m loosing a lot of hair.

So my question is, what causes hair lose?

I was stress a few days ago. I wasn’t eating as much as I use to for like 3-5 days. But now I feel less stress and my appetite is back to before. Could this be the reason?

Also last time I got blood work my physician said I have vitamin D deficiency