The left side of my lower abdomen is visibly more bloated (or more fatty) than the right side. what causes this? what's the organ located there?

Not necessarily : Unless you have pain or discomfort . Minor assymetry of opposite body parts is not unusual. However if you have symptoms or worried see a doctor.

Colon: If you are very skinny, can be filling prior to bm. Not much else would show up that way, and as long as not causing / having other symptoms, then not likely a problem.