You might also feel nauseous during or after a run because you're dehydrated. Nausea is an early symptom of dehydration . The current recommendations for hydrating while running is to obey your thirst and drink when your mouth is dry and you're feeling thirsty. In general, that means about 6 to 8 ounces of fluid for runners running faster than 8-minute mile, and 4 to 6 ounces of fluid every 20 minutes for those running slower than that.
During longer workouts (90 minutes or more), some of your fluid intake should include a sports drink (like Gatorade) to replace lost sodium and other minerals (electrolytes). And don't forget to rehydrate with water or a sports drink after your run. If your urine is dark yellow after your run, you need to keep rehydrating. It should be a light lemonade color.
Another possible cause of nausea during or after running is that you simply ran too hard and overexerted yourself. One way to avoid this problem is to make sure you are warmed-up before starting an intense run and to run at a pace that you're ready for.
Source: Maharam, Lewis, MD, et al. "IMMDA's Revised Fluid Recommendations for Runners and Walkers" 5/6/2006