What is wrong with my German Shepherd?

That is really hard to say. I would give him or her lots of water.

He'll certainly be fine till then

My dog does the same thing and turns out its just a bad case of acid reflux or heartburn.

I would still recommend taking him to the vet just to be sure however because (depending on the age of your dog) it could also be an early sign of heart disease.

Until then however you can try giving your dog a pepcid (available over the counter in any drug store)

These are safe to use on dogs as recommend by my veterinarian. That should either fix the problem, or rule out that your dog does have indigestion.

If it is getting worse or it's really bad, take him to an emergency vet. Is it worth the risk to wait till Wednesday?

Hooray for you! you have a male canines who does not pee on each little thing in sight! LOL I also have a male who does that too. yet while he around yet another male canines that he does not understand he will carry his leg. :) purely a theory - have you ever had his hips checked? My canines has undesirable hips and that is between the excuses he does not carry his leg. to no longer scare you, i could ask the subsequent time you're on the vet nonetheless.