Why would my lower back feel really tight after standing, or walking, for a while?

Back problem: May be some sort of issue such as spondylolisthesis/pars defect which might be common in your age group, or even something more concerning like ankylosing spondylitis or possibly the more common mechanical back pain. A doctor examination is the best place to start to sort out likely possibilities and if required arrange further tests. All the best.

Strain: Your low back muscles can get irritated even with regular activity. NSAID and rest. Warm compress. If no improvement get checked.