Why is my floor on fire?

My room:

Now occasionally, a random piece of my floor will catch fire for about 10-30 seconds:

I am aware of a lava pit immediately below this room, but I thought stone was supposed to be fire resistant!

This has already almost killed me. What's going on?

Solution 1:

Apparently fireballs from lava can tunnel through up to two blocks sometimes causing a considered-to-be-safe location to catch fire.

^{I had to rebuild my wooden floor three times before I noticed my lava fall outside was to blame. (Since it's SMP with bukkit, WorldGuard solved this issue for me)}

Solution 2:

Combining the inputs of Raven Dreamer, chands, Tobias Kienzler, Kromey, Kevin Y, and Kevin Reid, I have decided that:

1: I caught on fire because I was unlucky enough to be diagonally southwest of a lava block.

2: The lava underneath my sign is tunneling up to it.

3: But because of a bug (possibly because a sign isn't a solid block), the blocks around my sign have been catching fire instead.

Many thanks to everyone.