I started putting on more miles this year.. did about 1k mi last year and this year i'm at 450mi so far. I've been increasing mileage per ride and exerting more effort.

My lower back started hurting about 3 weeks ago and it was bad for about a week. I found that it was my hamstrings and glutes that were really tight and on longer/harder rides my form started to deteriorate as I got more and more tired.

My remedy was much better stretching and warm-up/cool-down. I've been fine ever since.