Always feel faint during and after having a bowel movement. it doesn't get better until i lie down or sometime not at all. what causes it? b/p normal.

Valsalva maneuver: Straining at defecation causes an abrupt drop in bp. Although you note that your BP is "normal", it's the fall in BP that causes dizziness and it's likely that yours falls with valsalva (straining) as does everyone's. Your lactose intolerance and IBS may have caused some volume contraction - you can counteract this by pushing fluids and liberalizing your salt intake. Good luck!