When I am nauseous, I actually provoke the vomiting. It have always worked for me. I would not listen to advises telling you to consume anything other than water, unless you are physically exhausted. Then electrolytes would be a logical step and/or interrupting the fast after a while.
History of congestion of the liver/gallbladder or chronic medication would also show as nausea, since it is another venue for the body to clean.