7 week old gagging for no reason?!

Morgan
My 7 week old son(exclusively breastfed) will sometimes just be sleeping or laying to get his bum changed & will just start gagging & almost starts choking .. For no reason?? My sister inlaw says her son does it, she thinks because he gets a bad taste in his mouth?? I had him checked today for acid reflex & my docter doesnt think he has it .. 
Has anyone else's babys done this ?? If so do you know why??
Thanks