I started trying to get fit about 6 months ago. I mostly bike (70 miles a week), but I sometimes mix it up with jogging (which I do using minimalist shoes; I used to not be able to run because of knee pain, but those shoes really seemed to take care of that). I'm a bit overweight, but not horribly so -- I'm 5'11'' and 190 pounds. No injuries that I know of.
Anyway, I went out for a jog this afternoon, and I had a weird pain. It started immediately. Every time my feet hit the ground, I felt my lower back start to tighten up (in a narrow band just above my buttocks, all the way across the back; if I press on the location, I feel what I assume are the tops of my hip bones). It got tighter and tighter every time my feet hit the ground, and eventually it was so painful that I had to stop. I made it maybe one block (0.1 miles) before I had to turn back. If I do very light jumping in place, I get the same tightening and pain, but I don't feel anything from ordinary walking or from biking. It is a dull and not a sharp pain, and I haven't felt any bulges or had the sensation of something "giving out".
Does anyone have any idea what is wrong? Are there stretches I need to do or anything?