Why did my hair fall out again after it just grew back?

I suffered hair loss in November. It took the entire winter for my hair to grow back and by May, I felt good about my hair again. I cannot believe my eyes..my hair fell out again in July. I am so devasted. It happened to me twice before over the course of five years but my hair always grew back with no help. Stress was always this trigger. This time, I took a vitamin called Viviscal on the recommendation of a derm. How could hair I just grew back fall out again? I never noticed excess shedding.