I've got a 2 year story with sciatica and lumbar spine problems. I'll spare you the long story and give you a place to start:

1. Ice, Ibuprofen, Hot baths and time off the bike are a great place for immediate relief.
2. See a doc. Get X rays and MRI's. Properly diagnosing a lumbar spine problem can be near impossible even WITH an MRI. Don't let your doc try to do it without.
3. Work with a physical therapist. In my experience, Chiro's do very little. PT's do the same things but also seek to strengthen muscles to correct the problem.

Likely, PT is going to be your best bet to get back on the bike and avoid future issues.

Good luck!

-s