Hm, sounds like you might be fighting something/coming down with something. To my knowledge there is no specific lower-back-pain virus, but when I get achy with the flu the pain often settles in my lower back/thighs. It goes away once the fever breaks, so if your pain is caused by a virus it'll probably go away as your body beats it.
This is how my dh started with the flu one week ago. He had terrible lower back and leg pain that we attributed to the fact that he had done some house painting the weekend before and was up and down the ladder. The day after the back/leg pain started he got a fever and chills. He never got much in way of respitory symptoms but the back pain turned into all over body pain that persisted for 72 hours and then he was fine.
I gave him lots of sodium ascorbate, rose hip tea, and beta carotene and he was fine within just a couple of days.
I've been wondering the same thing! I actually ended up at L&D for monitoring on Sunday b/c of the back pain and crampy feelings. I've been feeling generally blah for a few days - achy and tired - but not as acutely as when I've had the flu in the past.
So I guess I'm not totally alone in this, fwiw. Feel better soon!