I have clumps of hair loss, especially in showers, clogging my drain, why?

Previous stress: When hair loss is in clumps, it indicates that there has been a stressful event roughly 3-4 months prior that caused your body to signal your hairs to fall out called telogen effluvium. The stressful event can be emotional or physical. The classic example is postpartum hairloss. These follicles are still present. As long as the stressor is gone, your hair will grow back.

Hair: The are many reasons for alopecia. Medical reasons include certain medications, hyperthyroid conditions, and stress related. Some forms of alopecia are idiopathic and do not have a specific cause.