The hair growth is cyclical. It grows for some time (anagen phase), and then it dies (catagen phase) and remains in resting phase (telogen phase). The telogen hair comes off with very little traction, e.g, while combing and scrubbing. On the scalp, after a resting phase of 3 months, the dead hair is shed off (if not lost earlier by traction) and a new hair regrows. If all the hair were in the same phase, then at the end of a cycle all hair will be lost and the person will become bald. Subsequently all hair would grow slowly at the same rate and would be of same length. But this is not what happens; the growth of hair is asynchronous. Thus at a given time, one hair may be 1 month old, another 1 year old, another 16 month old and so on. This results in presence of hair over the scalp all the time; but there is continuous loss of about 100 hairs per day. In some individuals, in periods of stress (physical or mental), a large number of hair die prematurely. These are then shed off (in numbers greater than normal) leading to sparse scalp hair. This is called Telogen Effluvium and you seem to have a tendency for it. All these lost hair will come back spontaneously once the stress is over. You may be having some additional problem. Clinical examination maybe along with few investigations will settle the issue.