Possibly the milk thistle had worn off in 6 hours. I take it several times a day. On researching milk thistle, I found out that it slows down the Phase I detox pathway in the liver. That is how it protects liver cells. Phase I detox pathway takes prescription pharmaceuticals, over-the-counter pharmaceuticals, street drugs, pesticides, insecticides, and other toxins and basically coverts them to a more water soluble form. Unfortunately, the form these nasties are turned in to by Phase I is often more toxic than the original form. In some individuals the Phase II detox path is slower than the Phase I. When this is the case, the highly toxic form of nasties generates tons of free radicals, which damage liver cells by the way, while hanging out waiting for Phase II to finish disarming them. Milk thistle slows down the Phase I detox pathway so Phase II can keep up. This is how it protects liver cells. So it you get symptoms after 6 hours, I'd say that the milk thistle you previously swallowed has been used up. By the way, you may want to take turmeric as well. My research shows that it also slows Phase I, but at the same time it speeds up Phase II.

On the bottle of the milk thistle I am using, dosage instruction says, "Take 1 tablet three times a day before meals". Maybe having taken a full day's dose at once is causing you problems.