My Dog is not eating, and...........?

I have a 4 year old Shar Pei, and he has been in great health until this week. On Sunday, he threw up bile and little bit of food, which was no issue. Everything looked normal, no temp, little lethargic but nothing severe. On Monday, he was not eating and threw up brown poo smelling stuff. Took him to the ER vet right away, they checked him out, no bowel obstruction, no significant issues with his blood work. All showing that he stomach was upset but no blockage. It has now been 4 days since the initial incident and going back to the vet today. Does anyone have any ideas on what it could be? I am at a loss and it's killing me to see my dog in so much pain. Thank you in advance!