I just bought a Trek 7200 FX last Thursday. On a 14 mile ride today, the left crank arm came loose with about a mile and a half left. I tightened it by hand (no tools with me) and took it easy the rest of the way home. I had to stop a couple more times to tighten it again. Is this something that I can take care of on my own, or should I take it back to my LBS to be looked at?