The part I found odd when actually giving CPR was it seemed I was blowing air into something without resistance. It took a while before that seemed effective.
Our family doctor explained that you may actually be blowing air into their stomach also. I had not thought about that. It would depend on how the flap was working for the windpipe. He told me it was normal for the patient to have belching or gas afterwards due to the procedure.
It is not uncommon for the patient to vomit before, during or after CPR. That is one of the bodies defenses when the heart is in stress.
Not only does the rescuer have to deal with that, the patient, if they recover, may wind up dying of aspiration pneumonia.