Definitely should see a doctor to rule out any of the serious disease processes such as a thyroid disease, polycystic ovaries, or an autoimmune disease such as lupus. Less serious causes of hair loss could be a fungal infection of the scalp. A nutritional deficiency such as a lack of essential fatty acids, biotin or zinc can cause hair loss. When the body is under unusual stress one can temporarily loss their hair. This can result from high fever, childbirth, chronic illness, major surgery, severe emotional disorders, crash diets, and some drugs. Hair loss of this nature happens 2-3 months after the physical or mental stress. The disease of baldness that people are refering to is alopecia areata, which results in round or oval patches of hair loss. The eyelashes and eyebrows can also be affected.
One thing is for sure it is not normal for a 19 yr old to have her hair fall out to the point that it is looking thin or having blad patches. Some blood tests are definitely in order.
Suzr