I read the other day that diarrhea is an acute form of constipation - now this may sound crazy,but if your bowels are backed up with crud i.e. the lining of your intestines has old waste matter stuck to it. Waste needs to leave the body somehow and if this is the case, the body produces diarrhea to get it past the other stuff.
Otherwise it sounds like you have either some form of parasite or bacteria that shouldnt be there. I suggest you check out forums such as Ask MH, the Bowel Cleanse & Liver Flush Forum s as a start.
Bowel movements are the most important thing, so start reading!