For constipation I would recommend Acacia fiber at www.helpforibs.com. This website also offers a wealth of information to help you through your really tough days. It's fabulous! Acacia is the least gas forming fiber and offers effective relief from constipation. For the acacia to work properly, it's crucial that you start with a low dose (1/2 teaspoon twice daily) and gradually increase to a dose high enough to resolve constipation (which may be 4-5 tablespoons a day). If you got gas or bloating, that would mean you increased your dose too quickly. It can take several weeks to gradually increase up to the dose you'd likely need to resolve constipation, and unfortunately you can't rush this. Acacia is not a drug or a laxative so it won't work overnight.
I really feel for the constipation folks, as it takes some patience to start slowly, increase gradually, and eventually get your daily dose up high enough to resolve symptoms. A lower dose can't worsen constipation, but it won't resolve it, either. You may want to start over following these directions and see if you have better luck.
Senna tends to be a little too abrasive to the gut and therefore those with IBS or sensitive digestive systems is not the best choice. Senna also causes more gas and bloating for some, especially those with constipation.
Of course drink plenty of water.