It’s about blood flow. You’re exhausted, and so your body is struggling to pump it around the body. It’s been taken away from your intestines, where it’s needed for digestion and absorbing food, and as a result you’re feeling nauseous. It’s advisable to rest straight away.
What has happened is that you’ve gone too far with your exercise and effectively reached a point of trauma. That’s easy to do. We’re competitive animals by nature, and in a team setting, such as five-a-side football, or an intense environment, such as a crossfit or gym class, it’s easy to push beyond your limits.
Instead you should come to learn your boundaries and push them carefully. Don’t eat too much before you train, and figure out what eating times compliment your exercise. Build a routine before you start exercising. That routine will bring you confidence whenever you work out. And if you come to feel sick again the next time you train, work within yourself. Hold a few per cent back. Then push yourself again next time.