Sharp stomach and side pains when I eat lots of fiber, e.g. fiber 1 bars. Try not to eat as much but get constipated if I don't. Jw what the cause is?

Pain and Fiber Use. While it is difficult to determine a clear connection, I would review how much water you consume every day. If you water intake is below 8, 8oz glasses per day, this may lend to having constipation and stomach pain when consuming large amounts of fiber in your diet.