I have had instances where I have been in the middle of diarrhoea and suddenly felt like I was going to vomit. I attributed this to stomach cramps since I had no nausea before hand, it felt like I was being squeezed in the middle forcing everything up or down. These instances seem to happen when I have eaten something that disagrees with me (usually I don't have diarrhoea as a regular Crohn's symptom), or when I have had a stomach bug.
My boyfriend has had similar when he had food poisoning, and would rush to vomit only to find it coming out the other end too.
So it sounds to be a fairly common occurrence when something has seriously upset your stomach.