What's wrong with my dog?

Alright, his name is Max. The idiots that had him before us gave him heartworm. He went through treatment, recovered, and was doing alright. That's when we adopted him. He is older, although they don't know his exact age. Recently, he has gotten better at walking (he could barely walk a half-mile before), and is now walking one mile loops five days of the week (give or take). Unfortunately, he has also gained some weight, and while he weighed about eighty-five pounds when we got him, he now weighs about 115 pounds (I know). We've been working to help him lose weight by no longer sprinkling grated treats onto his food, as that was the only way that he would eat it dry (we used cans and dry food before this, hence the big weight gain). We've also eliminated the unnecessary snacks and food given to him. He's lost a few pounds so far. So anyway, today when Max woke up, he seemed tired. More tired than usual (he's a black lab). There was also a small spot of pee on his bed. Max has never done anything like this before, and has always had a bladder capable of withstanding long hours of not going. While Max usually follows you around to see what you're doing, today he just followed you and sprawled out. He has also been breathing heavily for short periods of time and has just appeared to have lost most of his energy. About three weeks ago, Max went to the vet. He tested negative for heartworm, and very other testable disease except for the one involving tall grass, which we didn't get. So, any help with this situation would be greatly appreciated as we are all worried for Max. Some additional details that may be helpful are: Max is still doing normal things throughout the day, just not as quickly or energetically as usual, and Max is also less interested in food (although it's hard to say because he hasn't liked food much anyway do to the lack of cans/treats on it),