just a hard life or Could this be some sort of injury ?

I've been a mechanic/maintenance guy for about 7 years now, and am pretty hard on my body, but in the past couple of years I've had on and off lower back problems (among other things). It started suddenly, I don't recall any injury to my back, but one day it just "went out on me". I was bending over to get something from the bottom cabinet and it felt like someone stuck a knife through my spine about 3 inches above my waist, I hit the floor, the pain went away immediately. When I do something strenuous, like moving furniture, bending over a lot or for long periods of time, it will hurt for a day or two and slowly get better. I am a little overweight but not terribly, I know this doesn't help since the pain also comes with bending backwards, like when moving furniture I lean back sticking my belly forward. The most recent occurance I did notice the low back pain along with lower abdonimal "muscle fatigue" I guess you would call it, along with a feeling like my bladder is very hot. I've had this feeling before also whic goes away after a day or two. Also when I'm welding, it's standing a certain way for several hours, leaning back slightly, which causes soreness. I can go in and sit down and slowly bend over because it is stiff and painful, but can eventually bend all the way down. Could this be some sort of injury that I wasn't aware of at the time and is getting worse, or could it just be the strain of my work taking it's toll?