If you have been riding a fair bit, you may have very tight hip flexors or hamstrings... both of these muscle groups pull on the low back. Personally i have extremely tight hip flexors that can cause extreme back pain while climbing. also what climbing position are you using. for myself, i am on the mtb, and i tend to lean forward, this is possibly the worst position for your back, it puts pressure on the vertebral discs as well as your back has to hold you in that position for the duration of your climb! Try to sit up more and stretch. Also if you work on strengthening your core this can also help reduce back pain.

hope this helps

Han