interesting, you mention you constantly stretched your hip flexors.
Another reason for lower back stiffness/pain is attributed to weak abs/obliques, having those muscles weak, forces the lower back to support most of the upper body by itself. It gets worse after heavy stress activities like soccer.
I'd recommend static exercises for your abs, 3 sets of 30sec static planks and side planks will do it for you.
Also, always remember to warm up properly before any sport. I get a terrible lower back pain after any soccer game, if I didnt warm up properly.