The last time that happened to me (very similar situation) I found out that it was due to the deadlifts. I had been making progress for months, then took a de-load (little easier work/rest)...then bam! lower back pain each and every time I did DL/squats (despite higher reps and good warming up). It's like I had built up tension in my lower back (via bad technique) and when I gave it a chance to rebound it "snapped".
Keep the lower back tight all the time! If this means finding a deadlift variation that stops you from rounding the lower back (e.g. reduces distance) then so be it. Some people have a build which makes it impossible to not round the lower back when deadlifting....for these ones I would recommend a closer grip, and wider stance. You could even deadlift with plates under the bar or something to raise it up more.
Currently, I'm doing a deadlift hybrid I made up myself - I do Romanian Deadlifts with the EZ bar (gripping the highest part of the bar which puts my arms inside the legs) and I stop the movement just past my knees (to stop lower back rounding). I take a shoulder width stance (again, the wider the stance, the less lower back strain). Get a good workout this way in all the right places, feel strong, and best of all, great feeling in my back (no problems since).