My pup used to do this in the mornings quite frequently. Peeps here suggested it may be because her tum is empty for a long time overnight. I started giving her a biscuit when I went to bed and it's not happened again.
Hopefully this is what it will be and not the tummy bug back again. :thumbup:
It's probably just bile. If there's nothing in the stomach it can irritate the lining so the dog sicks it up. Divide the food through the day so the stomach is never totally empty for long, that usually sorts it.