Stringy white stuff and pus in bowel movement. Chronic lower left abdominal pain. Extreme gas, popping grugling noises coming from stomach. Unable to eat pretty much everything! Especially dairy!!
Could be lactose intolerance. The Mayo Clinic has a good overview of that:
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.