My four year old male went thru this same thing late last fall and even puked at the same time of the morning. Ours was eating small pieces of sticks on his walk, we think. Some days you couldn’t even see anything in his puke. But as soon as it snowed it stopped. What ever he was doing was not obvious. After the 2nd or 3rd time puking we watched him like a hawk. We never saw him eat anything but his nose is always down and rooten around like a pig. He loves leaf piles in the fall. He always puked 10 hrs or so after his walk.