If you find you are a very heavy sweater, some studies suggest that the gut stops absorbing fluid, you become nauseated, and eventually vomit. This is especially true for those who are dehydrated before they start to replace fluids during exercise.
There are some people who vomit following intense exercise, especially when it is more extreme than your body is accustomed to. This could happen to runners who really push the last kilometre or two of the marathon.
If you feel this may be your cause of nausea, start by checking your prerun and postrun weight to get a feel for your sweat losses. You need to be careful to not exceed your sweat losses with unnecessary fluid replacement. A general rule of thumb is to just drink when you are thirsty. Do not push fluids when racing.
