My 4 yr. old twins have been vomiting clear frothy liquid and complained of stomach ache.  They do not have a fever and one of the twins has a runny nose. After they vomit, they go back to being normal 4 yr. olds.  It has happened twice. What could this be?
This might be a stomach flu - i.e. the virus playing around with the GI system in addition to causing havoc in the nose.