The mucus can be in your stomach (like food) because you may be swallowing it in your sleep (like Diane said), or even possible to swallow a little when you are just eat/drinking. Mucus from your sinuses can drain down the back of ur throat while u sleep causing it to collect in your stomach. When you wake up, do ur treatment, when u began to cough, you can set off the reaction of vomitting. Also, as Joni said, if you are eating close to ur treatments, it can also cause it. If you eat, then do a treatment that induces a lot of coughing (vest or HS) within 10-20 mins of eating, then it can cause u to vomit.