What can cause blood in diarrhea in a dog? (Background info included)?

I have a large breed German Shepherd mix. He's two years old and UTD on all his shots. I'm in college so we recently moved from a small town into a big city apartment. Since we moved, he's had very runny stools, which I was told by my vet could be caused by the stress of such a drastic move. I did... show more I have a large breed German Shepherd mix. He's two years old and UTD on all his shots.

I'm in college so we recently moved from a small town into a big city apartment. Since we moved, he's had very runny stools, which I was told by my vet could be caused by the stress of such a drastic move. I did buy a large garbage can that we keep water in at all times to ensure that he doesn't dehydrate from this, but I was told the diarrhea would probably go away on it's own.

This morning however, while we were on our routine walk I noticed that he had a two or three tiny (maybe about the size of a dime) drops of blood. I've called my vet and made an appointment but he can't see us until tomorrow. So for now, I just want to know what COULD be wrong with my dog.

He isn't acting any different. In fact, after the incident he wanted to play fetch and roll around in our sand volleyball court. His diet has been fine, but I did switch his food over a few days ago from the Purina Pro Plan to a cheaper brand (I don't really get paid a lot).

We go to the dog park once a week and I take him to the lake to go swimming often. He's around other dogs on an almost daily basis, so is it possible that he could have caught something from them?

What could be the cause of this and should I worry? Also, is there anything I can do or feed him tonight that could help him out? I'll still be taking him to the vet tomorrow but I'd love to hear any advice.

Thanks in advance
Update: He does lick his rear quite a bit, but he doesn't really show any signs of pain.
Update 2: He eats dry food only.
It's possible he may have eaten something sharp though. He loves chewing on things, especially pencils that I or my roommates accidentally leave in his grasp.
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