Health Question for 11 week spaniel

Question
Hi,
I have had Buster (purebred cocker spaniel) for a little over a month now. I bought him at a pet store, and since I had him he has had really bad diarrahea. He has been diagnosed as having Cocidid and Giardia. He has been on meds since the day I got him, and has finished them all. However, he still whines every morning when he wakes up (like he is in pain). He is also now vomitting white foamy stuff, and yesterday did something very odd. While he was outside using the bathroom, he all of a sudden fell onto his side and layed in the grass without moving. I approached him and rolled him over, and he was moving very dissoriented and couldnt move his back legs. This continued for a few minutes, then he ran inside, and hasnt done it since. I am not sure what to do. I take him to the vet all the time and they say the giardia and cocidid are just stubborn in going away. Does it sound more serious to you? Please help, Im not sure what to do.
Thank you
Danielle

Answer
I don't like the sound of things at all.  You need to talk to the vet about the falling down incident.  It could give him one more clue about what the real problem is.  If he doesn't start making some progress, perhaps it is time to see a second vet.