Drinking a lot and collapse.

My boxer, Tank, has recently began drinking water excessively and then pees all over the house. He has never peed in the house before. Even when he was a puppy he was very easy to potty train. We soon found out that some of the spots we were cleaning up after being gone was vomit (white, foamy, with yellow which I'm assuming is bile). He has lost about 10 pounds in the last 5 weeks and is not eating well at all. When we took him to our usual vet, they did some blood work and x-rays and said everything was normal but there was a possibility he had a UTI (which he said was very rare in a male dog). He gave us antibiotics to treat that possiblity. I asked when to really worry about him not eating and he just said he will eat when he's hungry. After giving him the meds he acted better, more himself, but still would not eat and is drinking probably at least 3 gallons of water a day. I don't know if that's normal but that is way more than he usually drinks. He is not urinating so much in the house as he was but does ask to go outside quite frequently. As I said, he seemed to be doing a lot better after the meds but when I came home from my sons baseball practice tonight he was barking from downstairs which is very unusual. When I checked on him he was sprawled out on the floor, belly down with all four legs sprawed out besided him. I looked like he had fallen. When I tried to help him up he kind of wimpered but once he was up, he walked around and went upstairs. He in now laying down, shaking, and looks like he did in the beginning. I know I'm rambling but we also have a border collie mix that earlier this week was having trouble walking. She was literaly walking on her front legs.It was like her back legs just would not work. She is doing ok now but I'm wondering if these could be related. If not, do you have any other ideas of what could be going on with Tank?

Thanks for responding so quickly. I don't feed the dog's chicken jerky treats (thank goodness, I saw the recall), and there have not been any macadamia nuts around the house. I looked at his gums and they look pink. It's kind of pale but still pink, no white. If it was a tumor, is it possible that the bleeding is off and on and that may be why he feels better for a while and then feels bad again? Or would it be a constant thing where he would progressively get worse?

Ugh...that's what I was worried about! Thank you for your help. I am going to try to get him to an emergency vet tonight or to a new vet first thing in the morning. Again, thank you very much!

Hello Dr. Marie,
I wanted to let you know that we took Tank to the vet first thing this morning. Turns out he has two large masses in his lungs. Our current vet said he recalculated his blood work and it showed that his calcium level was extremely high. They are leaning towards lymphoma He gave us a referral to a specialist so we will see what we can do next. Thank you again for all your help! This is a wonderful service you are providing for the public!!!