Why do I feel nauseous, have chills, and have gas after running a half marathon?

Dehydration, heat . Make sure you drink enough during marathons, and as well take adequate "electrolytes" (salt). Dress lightly, make sure your clothing is well ventilated, avoid occlusive clothing. E.G. Visor hat is better than a hat that closes off evaporation. Humans were designed without clothing, and when you are hot and exerting, try to get back as close to that natural state. Spray water, evaporation cools.