Back Pain! What the hell?

I need some advice, nursy type advice.
I think I have pulled a muscle in my back, but I'm not sure how.
I went to the gym yesterday, ran 5k, did free weights for my arms and lats. Came home, decided to clean out that stupid wardrobe of horror in the hall-which I've been threatening to do since last year, I mean do I really want fish scale pants circa 1994 or a bag of moudly old dish cloths? Or eighteen yards of paisley fabric-dont ask. Brought the unwanted crap to the bin. Put on a wash, cleaned out the old cat's ears, watched Dog the Bounty Hunter and went to bed. So far so dandy.
So how come when I got up this morning I feel like someone kicked me in the kidneys. It hurts if I turn, it hurts if I lean, it hurts sitting here in my chair...right hand side, in the dip, you know, below the ribs just above the waist line-I know, so technical.
Anyone have any experience of back pain? I've never really had one before, what might have caused this? Etheline says not to take pain killers as they might mask the pain and I might do more harm. But that doesn't make sense and I suspect Etheline is a masochist. I have Spanish painkillers in the kitchen that are so strong I could amputate my little finger and grin about it. I want to take two of them and lie down, or at least numb the pain so that I can work for a while. Anyone know what to do? Me love you long time if you do!