You may be suffering from alopecia (don't freak out, this is very common during and after pregnancy, it may come with age, or you could have a fungal infection). Your hair is directly effected by genetics, your diet, and how you take care of it! Maintaining a healthy diet and caring for it properly (not using harsh chemicals- y containing ammonia, bleach, etc.) should help it grow healthy. To speed up the process of its growth don't be afraid to take prenatal vitamins, these can be taken even when you're not pregnant or nursing!