Can't help you on the flax--I don't notice any intestinal effect from the caps, take 10 a day, and don't get headaches.

I take a lot of fiber caps as needed; they tend to have more of an overnight effect than anything you're going to notice right away. If you need immediate, try one of the salts (but make sure you can afford the risk! No long drives...). When the system gets really bad, I've taken as many as two fibers every two hours with LOTS of water until things get back to normal. (I'll stop up when I change time zones, every time--the trip to New Zealand was as intestinal nightmare.)