What just ran through my water pipes?

If you are on city water, I would guess there was a break in the line that was repaired, but there was some dirty water in the line from the process. It just had to flush through, and all your neighbors probably expirienced the same thing. If you are on your own well, there could have been a little cave-in, no big deal. The egg like smell in a natural well comes from mineral deposits, like sulfur and stuff, but in a city water system, it could have come from welding, or whatever they had to do to repair the line.