Baby vomiting during the night only

My 7 month old lb has started being sick about an hour after his feed at 4am. 

He's done this for 2 out of the last 3 mornings. He had a couple of loose nappies yesterday but 48 hours between vomiting episodes seems odd if it were a bug. 

He was off his food but not his milk on Thursday when this first started but is back to his usual self and taking food and milk.

When he vomits it's proper vomit that smells like real vomit not milk. 

He keeps other feeds down and has only just started this. He is on anti reflux milk and under a paediatrician for his reflux.

I have no idea what's wrong with him....has anyone experienced anything like this before?

Hope someone can give me some ideas as to what's going on. Thank you 😊