Yep, OS - that's why. The first time it happened to me, I was worried sick - did I finally catch something nasty? OMG!! But, nope; figured out that the pH of semen can sometimes tip that delicate balance of water in your gut in the "wrong" direction and... bring a book or something with you as you haul ass to the bathroom. I even discovered that guys who drink on a "more than regular basis" can cause a change in their pH and when you swallow, you get the run just like they would if they drank more than their gut can handle, tipping the balance over and giving them the runs.
I added this factoid for all those guys who've ever gone out drinking and found themselves with the runs - and especially if they get up the next day and drink some water. Sometimes, ya just gotta know why things work the way they do so if ya didn't know this, now you know!