Feeling sick after working out is fairly common and can be caused by several factors related to hydration, nutrition, exercise intensity, and environment. Here are the main reasons why this might happen:

• Dehydration is a primary cause. Losing too much water through sweat without adequate replacement can lead to nausea, fatigue, and headaches.
• Low blood sugar from not eating enough or improperly timing meals before exercise can make you feel queasy.
• Eating too soon before a workout or consuming foods high in fat or protein can delay digestion and cause nausea.
• Blood is redirected during intense exercise from the stomach to muscles, impairing digestion.
• Lactic acid buildup during high-intensity workouts can contribute to feeling sick.
• Jarring motions like running or jumping can physically upset your stomach.
• Exercising in hot conditions may cause heat exhaustion symptoms including nausea.
• Overexerting beyond your current fitness level may also trigger nausea and dizziness.

In summary, feeling sick after exercise often relates to how well the body is fueled, hydrated, and conditioned, as well as the workout environment and intensity. Adjusting these factors can help prevent nausea after working out.