I do not think the pharmacist was
correctCorrect (new formula) to blame the Zyrtec.
The diagnosis most likely to cause hair loss in the pattern you describe is alopecia areata. This is a common cause of patchy hair loss. Cause is unknown, though the condition is referred to as "autoimmune," because the body seems to be attacking itself (the hair follicles.) The hair is dormant but not dead. Regrowth is possible, though it may take months. Treatment is available.
Though alopecia is often glibly blamed on "stress," the asociation is tenuous at best.
My advice is to see a dermatologist for definitive diagnosis and treatment. The chances are very great that you will "wake up bald"--the condition usually stays patchy at worst.
Best.
Dr. Rockoff