Psyllium WILL constipate, especially if you are prone to constipation in the first place. You can try adding a magnesium supplement to the mix. But I'd really recommend that you stop the P & B shakes, that you use magnesium or Oxypowder or Colosan etc. to get your self moving first, and then add the P & B.
You can follow the Oxypowder , Colosan etc. directions or you can just take Magnesium Oxide capsules (I like Twinlabs, and every health store sells them) right before bedtime. I'd start with two and then if you don't go in the AM, take three the next night, until you reach the dose that cleans you out.
I know this isn't my thread, but I've been through this before. :)