Baby trembling/shaking while nursing to sleep

Last night was the second time my 4month old has fell asleep while nursing and started to tremble all over. I freak the crap out and sit him up right, and he stops and wakes up. Last time he did that was about a week ago, and it was also once he fell asleep while nursing. We have a drs appt today but I can’t stop thinking about it. I couldn’t sleep at all last night. Has anyone experienced this?

I know it wasn’t a real seizure because he stops immediately when I sit him up, and looks at me like what the heck. But I’m so worried.