Loose stools are one of many pre-labor signs you may experience. Toward the end of pregnancy, your body releases prostaglandins that help up soften your cervix.
Prostaglandins also soften bowel movements. While this may sound yucky, keep in mind past generations of mothers were given enemas when they started labor! Loose stools are typically nature's way of clearing things out.
You mention, though, that you've had diarrhea all week, so just to be safe you may want to call your doctor or midwife to rule out a stomach ailment that's not related to pregnancy.