I went on a ride today and half way through I felt/heard my crank make a "cracking" noise, but I thought I just ran a twig over or something. As I was riding back home I felt my crank slip, and a second time when I almost got home. I know the typical answer is to tighten your cog/lockring, but this happened when I was on my Singlespeed freewheel. Any ideas why? I plan on taking my bike to my LBS soon, but I just wanted to get a general idea of what was wrong.

Thanks!