What could it be?

My 6 year old keeps having random bouts of vomiting. He’ll be fine for days or a week or two and then he’ll have a day when he throws up several times and is super tired all day and falls asleep at like 5 (usually bedtime is 9 and he fights it). It’s not a stomach bug because it’s been going on for months now. It doesn’t seem to be triggered by any particular foods or activities. His doctor said it was probably reflux but the medicine isn’t working.

Anyone else deal with this? Any ideas what it could be?