Lower back pain when bending neck?

Couple of things: Sounds as if either there's traction on the cord sometimes referred to as a tethered cord...or you may be experiencing a funny thing called Lhermitte's sign though your description's not classic. Have you recently developed SEVERE migraine like headaches by chance with fever? Hopefully not..just a question. In any event, any of these possibilities deserves a workup by a doctor. Cheers!