Chihuahuas

I recently got a "Teacup" Chihuahua, his name is chico. When I got him they said he was 7 weeks old and he was eating, playful, and everything was normal. I took him to the vet a week and a half later (should've been 8 1/2 weeks) the vert said he was only 5 weeks, 6 weeks at most. They gave him his first shot. 4 days later he had a fever, diarreah, whimpering and was lethargic and he lost some weight. Then the next day he was fine, and was fine for 2 days. He doesn't have parvo and I don't know what to do now. After those two days he got sick again and it has now been a week since then and he has been getting progressively worse. He now has a loss of appetite and thirst. And when he poops it is really runny, mixture of blood and yellow stuff, whimpers when he tries to poop, but is swollen and tender, tender tummy, and he always is trying to poop and sometimes can't. There is usually some amount of blood that comes out though and sometimes it is only a drop. Some one please help me, I don't know what to do and he is the only one I have. I know it might sound weird but he is like my kid, he is always right by my side and we have become very attached!!! Please and Thank you in advance! natasha and chico