It could be many things to be honest. But deadlifts and such symptomatic behaviour might indicate a slight bone malalignment such as a Wikipedia reference-linkspondylolithesis. In terms of inexpensive investigations a plain x-ray might help to confirm or rule that type of issue out. These things can happen from a small pars fracture on a vertebra which is not uncomment in sports. There are many other causes or referred back pain too and of course if you search the internet on all those you might thinking the end is near, as such symptoms (like drugs side effects) can come from most systemic problems too. So be wary not to self diagnose.

The best way forward is to get some advice following a full physical and movement examination. Go see a good local physiotherapist or osteopath or even a chiropractor to see what they observer through a proper assessment. Let us know how you get on.