Why do people vomit if they exercise too hard?

Physiology: Vomiting is one way of telling the body to slow down and take care. Fluids are needed and and rest is needed. Toxins build up in the body with aggressive exercise and pending the degree of conditioning one is in. Lactic acid builds up as well as other toxins. Hydrate, hydrate, hydrate. Some are just more sensitive to stress than others. Take it easy!

Blood flow, chemical: Exercise moves blood out of organs & into muscles, sometimes so much so that the organ doesn't work as well. This is why you shouldn't eat a lot before exercise. Hard exercise also releases a lot of chemicals in your body and dehydrates you. Best way around this is to stay hydrated & not work harder than you are trained for at the moment. Always warm up & add intensity gradually.