What everyone else said about going to the doctor.

I had a herniated disc a few years ago, and still have some residual discomfort, and muscle spasms. A couple of things you can do to reduce the pain are stretch (especially before doing any physical activities like riding a bike), take ibuprofen, and apply an ice pack, or heat pack to your back. In the long term strengthening your core should also help. Google stretches, and core exercises specifically for back injuries as some core exercises put a lot of strain on the back and may cause further injury.

You may also want to try raising your handlebars, and bringing your bike to a shop to make sure everything fits proper.

And of course go to the doctor, back injuries are nothing to mess around with.