system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does running sometimes make you nauseous?
gpt
It activates your sympathetic nervous system (activates squelettal muscles, dialates pupils and bronchioles) while supressing your parasympathetic nervous system (supresses digestive system, among otuer things). 

While you run this system change also changes where the blood flow is prioritized. So prioritize your muscles rather than your stomach. If you have food in your stomach and your system is no longer prioritizing digestion you may feel nausious because your body is redirecting its resources.  

This is simplified.