See your doctor
Dr. McAndrews is a dermatologist who specializes in hair loss and hair restoration. He advises patients in a situation like yours to do a hair count for 10 - 14 days. To do this, you gather up the hair you shed each day from brushing, styling, shampooing etc and count it. If it averages 50 - 100 hairs, this is a normal amount of shedding and not a concern. If it is more, you should see a dermatologist.
Shedding conditions can be caused by stress, poor diet, and a host of other physical, emotional and mental issues. If it is a physiological issue, then the dermatologist can refer you to the right person to address it. If it is emotional or mental (stress), then there is no direct treatment for the hair loss. But the good news is with the great majority of shedding conditions, you will eventually return to normal shedding, typically within a few months, and within a 6-12 months, the hair that shed prematurely will regrow.
So, see a dermatologist. (Look for one that treats hair disorders). In the meantime, do the hair count. It will help you to see what's going on, and will be helpful to the doctor for his/her evaluation.
Hope that helps, but if you have other questions, let us know!