For the past 3 weeks or so, I have had a dull pain in my mid-back on either side of my spine. When I "flex" this part of my back, almost like taking a deep breath, I get the pain.

My first thought was squats. I wondered if maybe I was leaning too far forward and this created the pain.

Maybe Deadlifts - same idea. Maybe I wasn't skidding my shins.

Or, maybe its my mattress. We have a rip-off temper-pedic cover on top of a normal mattress that's fairly old. We need to get a new one.

I even stopped lifting for a week to heal up, but the pain is still there.

What did I do?

Can you remember having any little pains or aches after a certain exercise before this dull pain began?

When I injured my shoulder I wasn't sure which exercises hurt it until I remembered having a wierd feeling after doing some incline pressing

It turned out that that wierd feeling became much worse than expected and resulted in a length layoff from pressing exercises

Not really. I did have some pain in my right lower back, but nothing else..