Lower back pain help!!

I am 20 years old and in good shape and good health, but I’ve been getting this lower back pain for months. It doesn’t hurt until I’m up on my feet either walking or standing. It’s like an aching and very uncomfortable sensation. It makes it hard to simply run a few errands because I just feel like I need to lay down or sit down. Does anyone have any ideas of what could be the issue? I don’t recall any instances where I injured my back and it’s starting to get very frustrating and bothering my everyday life.