Please help? is there a problem with my dog? her hair is falling out.?

My dog 11 months 50 pound shepperd/lab mix sheds a lot, when you rub her about 10 stands will come out. today i notice a spot on her backside that look like a lump. so i rubbed it. i notice it was hair. so i gave a little tug to put it back in place. when i did a hand full of hair came out. about 100 or so strands all with one little tug, i did it again, same thing happened. is this normal? she has never done this before