So sorry you are going through this. My hair thinned after having my second and fourth babies, and didn't all come back. It is actually a lot easier to style and curls better but I was vain about it being thick and big, you know.

I definitely think if you are in your 20s and losing hair then there has to be some kind of medical cause, it shouldn't just fall out. And birth control pills, starting and stopping them both, can definitely throw your hormones out of whack which can absolutely cause hair loss and I am not at all sure it would be found in tests.

Mine falls out less now that I do daily rinse-and-restyle, gave up sulfate shampoos, and take Floradix for iron, I'm not sure which is most important, but all are healthy so I just keep at it. I would estimate hair loss at about 20 hairs/day, which is sustainable and it isn't thinning anymore and feels slightly thicker though not like before it fell out.
2c / C / iii (FIA)
Med-Coarse, LowPorosity, NormalElast (LCLF)

Shoulder-Length now.

Wash: DevaCare Low Poo
Detangle & Rinseout: Walgreens or Sally's Conditioning Balm (always)
Leave-in: DevaCare OneC (budget choice is GFTN)
Style: ReCoil & DevaCare Arc Angel (budget choices are HESMU or Got2BSpikedUp)

Henna!