I am 38+2 and having the same problem
Thats one of the symptoms to going intolabor
Having diarrhea could mean many things.. You could have a slight tummy bug, the food you're eating may not agree with your stomach, or it's your body's way of preparing for labor. You can have diarrhea for up to a few weeks before labor starts. All that lovely build up has to come out at some time. Things are slowed down in there as your pregnancy progresses and you start to grow.
However, I will say that when I was headed into early labor with my second baby, that's how my labor started. Just kept having diarrhea and eventually started getting "slight" cramps that felt like normal ligament pains or the kind you'd get before your period. Also, nauseated eventually set in after a while and I started to get sick. But, no labor is always the same. Just how mine started last time.
I just woke up with diarrhea. I'm 29 weeks. I don't remember it starting this early but I suppose it could be any of the reasonings above. Hoping mine is just food related though. Good luck if this is heading you into labor! :]