I had been having tightness and soreness issues in my lower back that rolling, heat, Icy Hot, and any amount of stretching wasn't helping. The onset came when I started hitting a heavy squat period in my programming AND spent a lot more time sitting in the car due to a move.
A friend of mine with the same issues told me to start stretching my glutes more, like ridiculously more--the first couple of days I did maybe twenty minutes a day of just glute stretching, holding each stretch for five minutes before switching. It has worked great. Perhaps try stretching out the glutes and hamstrings? His theory was inflexible glutes were pulling the lower back down and preventing a proper lumbar curve.