I went through this exact situation about three years back, and i know just how scary and unplesent it is! First things first - go see your GP, because losing your hair may mean you are lacking in iron, or your thyroid is playing up. Basically, seeing your GP will put your mind at ease about any health issues. a lack of protein in your diet can also be the case
Another thing, understand that you're not going bald and even though your hair may be going thinner it wont all fall out. Apparently, every seven years your hair has a little spasm and starts falling out for no reason at all. The more you stress about it, the more you'll see your situation as scary, when its very normal so dont worry. Your mum might be right in saying it's stress, it could be that.
Try not to brush your hair excessivly either and dont keep it tied very tightly either. hope this helps