I don't know about the birth control problem but I had the same problem. Went to see my dermatologist and in my case it was a minor illnesss+stress+ history of hair loss in the family, can't remember the name, so I have to spray a product on my scalp for 3 months. And do this treatment once a year. My hair has already started to grow back. Not going to lie, it's going to take a while for it to grow back (normal speed).
I know iron defiencies and some food defiencies can also cause hair loss. But in any case, I would see a dermatologist, they are the specialits for that.