It could be due to various reasons. Don't neglect it. Pain killers give only temporary relief.
Some of them are: a slipped disc
, due to bad sitting position, tumor
Go to a qualified doctor (Ortho). Take Xray.
I had back pain
for two-three years. Never considered it as a serious one and never shown to a good doctor, though I had pain. Finally, when I checked with a good doctor (Ortho) and taken an MRI (from a good place), it turned out to be a rare, incurable disease, Paget's Disease of bone.