Could be lactose intolerance. The Mayo Clinic has a good overview of that: http://www.mayoclinic.com/health/lac...530/DSECTION=2
Could also be infection by the Giardia parasite or some bacterial/fungal overgrowth. If it's just bacterial/fungal it may respond to probiotics like acidophilus. Acidophilus may help with lactose intolerance too. You might want to consider seeing a doctor if you've had this for more than a couple weeks. The parasite/bacterial/fungal causes I've mentioned can be detected in a stool sample.