Amstaff puppy hurt or sick?

[Amstaff]

Hi everyone, I am new to this forum and hope someone can help me out. I
have a 3 1/2 month old American Staffordshire puppy who is the sweetest
little puppy I have ever owned. About a week ago I noticed he was slightly
limping on his back leg, but it went away. This morning when he got up he
was perfectly normal running and playing. Then I noticed he was walking
funny, favoring his left hind leg and he started whimpering at me. He is not
exactly limping he is just walking funny ( It looks like his back hips are stiff or
sore) After a while he didn't want to walk at all and lied down. I walked over to him and sat beside him and than noticed he was peeing while laying down, which he didn't notice at all, though it got all over his stomach. Now when he walks he pees sometimes and he is peeing a little bit more than usual. He has also started to chew on his stomach and penis while laying down. Also his hind legs have started to bother him even more, he kicks up the back one when he is walking like he wants to scratch his stomach or something, than he gives up and collapses on the ground. He also turns and snaps at his back legs when walking and runs like he is scared or something is bothering him. When he stands it is either with his back legs right together or with his head swung around today his rear end so his body is kinda twisted. He is still drinking and eating but he doesn't want to play or walk. I am at a loss what to do since I cant even phone my Veterinarian till tomorrow and my poor pup is so uncomfortable. If anybody has any ideas as to what could be wrong with him at least I would have a idea what to expect going into the Veterinarian tomorrow. Sorry for the length of the post.

[Amstaff]

Update: My pup, Diseal, threw up in my bedroom. He appears to feel a little better and is a little more alert. However he still does not want to walk. The only way he will lie is with his legs straight out behind him. I fed him wet Beneficial dog food today, which he threw up, maybe that had something to do with it?
I forgot to mention that he has his first two set of shots and his 3rd is on the 17th of this month

[Whizzy]

Boy, you've got a real grab bag of symptoms there. I'd suspect a possible kidney/bladder infection, because of the peeing. And a kidney problem could cause enough discomfort that he'd be reluctant to walk, too.

Another possibility would be a spinal injury, again because he's having peeing problems and pain in his rear legs.

It's definately something that needs to be sorted out by your Veterinarian, and the sooner the better.