What do I do about my sick pom. Dog?

My pom (about 1 yr. Old) is very sick, he became weak since yesterday, he hasn’t ate, and he refuses to do so. He only drinks water, he wont go to the bathroom, he doesnt want to walk, I adopted himfrom the miami dade animal shalter on dec. 28 and he was neutred before he was given to me. It seems like he was used tohouse food because every time I would eat around him he would stand up on two legs and do a turn to see if I would give him food. Which I wouldnt. I’ve been trying to feed him dry dog food mixed with somewhat moist dog food. He’ll nibble on it a little but lately he hasnt at all. He has also been breathing very heavily and fast. Idont have money to take him to a vet so I took him to petco to buy some medicine or something to give him some appetiate and nutrience but soon after I gave him that he threw up. I really dont know what to do, he wont eat, he wont walk, he wont listen, he wont do anything, im very worried. Whats wrong with him? What should I do?