Puppy piddling unvoluntarily?

Hi everyone,

I'm kinda worried about my labrador retriever. He's just under 8 months old and he's very well trained. He sits and stays for his food, walks on a lead happily enough, and best of all when he needs to pee he goes to the back door and sits. If we don't see him he'll jump up and whine until we notice he needs to go.

However, recently he's been panting and drooling like he really needs water and I give him as much as he needs. He cut his side deeply a few weeks ago and got surgery, so as a treat for being such a trooper we give him a hint of milk in his water and he loves it. He took five bowls last night, and I'm not sure a love of milk really stretches that far!

He's eating okay too, as far as I can tell. He wasn't eating at all but then we switched his food and he now eats fine.

Now, here's where the problem comes in. We were playing indoors today and he piddled right on the floor, but he didn't squat and he didn't seem to realise he was doing it. I put him outside but I didn't scold him because it was just a little accident.

Then about an hour later, he piddled again, but this time he ran for the back door, as if he was trying to make it in time. He tried to hold it in as he ran and when I opened the door he ran outside and peed for a solid two minutes. I'm really worried about him. Please help!