How long have you been riding? It might be a case of just strengthening your lower back. Make sure you aren't pushing big heavy gears. Very tough on the lower back.

+1 on seat adjustment, bar ends & private pilate lessons with a certified teacher. You can really mess things up in a big class where they aren't watching you. Pilates is very subtle. You learn to engage your inner core muscles, not just "suck in your abs." A few sessions is well worth the $$.