Losing alot of hair during/after shower - could it be the shampoo?

I never lost hair until a year and a half ago, then i noticed hair falling out. But most of it happens when i shower. The rest of the day the ammount of hair lost isn't something i'd even be concerned with, but sooo much comes out when i shower it's scary. Could it have something to do with the shampoo?

I'm 18, i've also been taking propecia for about 9 months now because the men in my family(father, uncle, grandfathers) have a history of going bald early, so i figured i might as well start now.

Thing is if i don't wash my hair for a day i don't notice a large ammount of hair loss at all. But if i take a shower and then brush my hair the brush is filled with hair, and so is the back of my shirt(i have relatively long hair)

I understand i could be losing some hair due to my genes, but it seems like i lose alot more after showering than i do throughout the entire day.