Dry Heaves More Condition_symptoms Why Or How Does Dry Heaves Occur?

Why or how does dry heaves occur? - dry heaves more condition_symptoms

I woke with a terrible headache for about 4 hours. Later I began to vomit, I felt so bad that I missed the bus and stayed home from school, I feel much better now. I hate missing, how can I prevent that, how and why this happens?