Edit: Assuming on an empty stomach from eating the night before.
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Several things. Exercising intensively can require a longer-lasting and powerful metabolism in the muscles to generate energy, which can eventually create lactic acid, a product that can build up in the blood, creating a state of acidosis (more acidic blood), which can lead to nausea and vomiting from triggering the area postrema in the brain (involved in detecting toxins in the blood and triggering vomiting). Dehydration, drop in blood pressure are other causes.
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