I experience a similar trend during the initial workouts of the 15's. This can be from a variety of occurences: One being excess lactic acid fermentation; this can lead to nausea. However, may I ask if you eat anything pre-workout?
Over many anecdotal experiences, I have noticed that (very non-physiological) 'Gatorade, Propel Water', sipped when needed, can easily cure this 'crap' or nausea feeling.
Good luck!