I read that metformin causes diarrhea but i have constipation. what's going on?

Carbs vs fat: Metformin decreases the absorption of carbs from your GI tract so if you eat a lot of simple carbs they will stay in your intestine, hold a bunch of water and cause a lot of diarrhea. If you are avoiding a lot of simple carbs (which you should) and you eat a diet high in fat (which slows your intestines) you will end up severly constipated. Increase you fluid and fiber to replace the carbs.