You should always report the injury immediately to your employer...even if you have no intention of filing a claim at the time. Injuries can get progressively worse over time, and then you could lose your job or get laid off and wind up without anyone to file a claim against, since your former employer has what is called a post-termination defense.
File the claim immediately. Find a good lawyer to send you to the right doctors and protect your interests through out your claim. Having a lawyer is not an act of disloyalty to your employer, but smart business to protect yourself from the insurance company adjusters and their own lawyers, who you will be dealing with throughout the claim.
You should file your claim as soon as possible to insure that you get proper medical care and to confirm that the injury indeed occurred on the job. The prior back issues will be apportioned through a doctor's final report which will determine what percentage of your permanent disability relates to the new injury.
I recommend you file the workers' compensation claim. Although it may not seem like much of an injury now and it may be related to an earlier problem, workers' compensation should be responsible for the portion of the disability that is related to this injury. If things don't get worse, you can drop the issue, but if things do get worse, you have protected yourself. Remember that workers' compensation is not a law suit against your employer...it is an insurance claim, no different than Blue Cross or Kaiser.
At some point, you may want to consult with an attorney. Find a good one here at www.avvo.com or at www.caaa.org. CAAA is the association for attorneys who represent injured workers here in California. Good luck.