There is a chance that the fiber plus compacted fecal material that has traveled down with the fiber has lodged in your colon in a way that it is irritating the sciatic nerve. The lower back pain running down your leg is a classic sign of sciatica. I have a bad case of it too and am doing the Oxypowder cleanse to try and remove any obstruction that might be keeping the nerve irritated and inflamed.
Try and do a colon flush with a strong Epsom Salt drink. Take one tablespoon of Epsom Salt in 6-8 ounces of applejuice or grapefruit juice. Drink plenty of water before doing this, stop water for thirty minutes before the Epsom drink and do not drink for 20-30 minutes after. You should see results within two hours-usually less. Stay off the psyllium for a day or two and see if the pain goes away. If it does, then you may need to consider doing a magnesium-oxygen based cleanse first to clear the bowel. An enema may prove helpful too.
Psyllium is a wonderful cleanser for people who are regular and have strong, naturally produced peristalisis. If you have a history of laxative use, have slow transit time, or an inflamatory bowel condition, then you may need to begin cleansing in a different manner than psyllium. I had to learn this the hard way, as have many others.