My experience is exactly the same. I take Flexeril before bedtime whenever I have lower back pain. It always works. I think what happens is that muscles cramp in my lower back and put traction on the nerves, causing pain and more cramping in a viscous cycle.
That’s exactly my experience, though I don’t take it at bedtime necessarily. I do give myself time to sit on the couch and do “nothing”. I’m not good for much when I’m feeling that pain. It makes me slightly drowsy at first but then I just feel relaxed and like mush after that. Better to just wait it out than try to do anything like work work. I probably do more leisurely reading when I take it, or hand sewing. When I feel that pain, it tells me I’ve overworked and that it’s time to slow it down a little. One day is usually enough.