I went for an 8 mile run today after 2 days rest, and half a mile in I had an experience I've only had 3 or 4 times before in my 3 years of running. I suddenly got a pain in my lower back. Kind of like a strong ache/spasm. I ran through it, like I have in the past, and after 3 miles it disappeared. What could explain this? When it happens, it's almost enough to make me want to stop, but if I keep running it disappears and I never feel it after the run or the next day. Is it just a random thing that happens? Maybe tension or something? Anyone else get this?