Lower left side tenderness before bm. Why?

Constipation? Are you constipated or having diarrhea? Any blood in the stool? Sounds like you need to see your doctor to get this checked out. A gynecologic problem may present this way as well.
Colitis. If colon is inflamed will hurt will fecal matter passes through - use choline or phosphadylcholine with bromelein to relieve pain and limonene 2-3 drops with water sip whenever you feel pain.