The star announced show cancellations yesterday due to what seemed like breathing problems.
According to the Miami Herald, the situation is much more serious. A source stated, "He is currently at a Miami hospital with heart issues and could be laid up for a while."
Williams was performing his show, Weapons of Self Destruction, in South Florida when he suddenly felt ill. Since last September, he has visited 80 cities to perform his stand-up comedy act to sold-out audiences.
In 1998, Williams won the Oscar for best supporting actor for his role in the film Good Will Hunting.