Why do I see so much hair in the drain when I shampoo? I am under a lot of stress at work. Female, 48

Alopecia. There are many potential causes including: hypothyroidism, menopause, autoimmune disease, vitamin/mineral deficiencies, sleep deprivation - if prolonged, and various medications or even over the counter medications you are taking. Consult with your md.
See your doc. Hair loss can be from dietary deficiencies, e.g. Iron, protein, hormonal changes, e.g. Thyroid, female hormones, aftermath of physical or emotional illness or stress, local scalp or hair issues, underlying medical issues, e.g. Lupus. Pattern of hair loss, e.g. Diffuse, frontal, crown, may indicate different causes. See your doc.