Exercising too much?

I was just wondering if maybe I'm exercising my dog a little too much. I try and drag it out and do some training in btwn so she's not just running like mad. Throw the ball, have her bring it back then sit and wait for a minute til I tell her to go again. And water is available for her at all times. (It's actually really cute when she's sitting and waiting for me to "ok" her to go - just raring to go get the ball - lots of false starts and flinching!)
So my question is it normal for dogs to have a loose poop after exercising? For instance at our lunch time walk she'll do a normal one before we get to the park, then after our 15 or 20 minutes of playing fetch sometimes she'll also do a not so normal one afterwards too on the way back from the park.
It's only after the free run fetch sessions that this happens. Is she too excited and over exerting herself? She always seems like she's having a blast and smiling :-)

And btw - boss hasn't said anything about leaving her at home yet so she's still coming to work with me! Yay!

The doggy could have loose stools from being too excited. But it could also be that the first stool is only hard because the bowel has been working on it but the second load passes right through because the doggy has no reason to hold it in. This happens often when a dog has mild diarrhea overnight and you don't let him out and he just holds it. In the morning it can look normal, but the next load is diarrhea. (A guy at our park had a dog who was allergic to his food but it took us forever to get him to go to the vet because he was all, "He's fine in the morning".)

Dogs at our park get diarrhea from getting humped by other dogs. When dogs get agitated or upset it can bring on diarrhea.