Promoted with no pay increase?

I took a job as a clerical assistant in a get successful fast food franchise. The pay was not great, but neither were the expectations. After a few months, the president and owner asked me to become his executive assistant, and I would also report to the vice president as well. I received no pay increase, other than the company wide annual 3-4% raise. My responsibilities tripled along the stress level. I've since developed and am being treated for anxiety. My boss, the president is a demanding man, whos famous for yelling at his employees, including me. I've gone home in tears on countless occasions. I researched executive assistant salaries online, and mine doesn't even register in the bottom ten. Meanwhile, my boss is multi-millionaire, with homes abroad and cars that cost quadruple my annual salary. My question is, was it my responsibility to ask for an increase when I was promoted, or is this company taking advantage of me? At this point, after the way he has treated me, I don't think a salary increase