What could it mean if I am experiencing hair loss as side effect of medicine?

Hair loss is listed in the side effects for the medicine amitriptyline that I am taking. I was just wondering what could cause it. The sheet that I received with it said to consult doctor if you experience hair loss. I'm not, like, losing chunks or anything, but I keep finding a lot more hair all over the house. And when I am in the shower there is a lot more hair falling out when I run my fingers thourgh is and wash it.