Ok, no laughing please... butt cramp?

Near the end of my trainer ride last night, I developed a horrible ache/pain right at my tailbone.  Was bad enough that I had to stop and get off the bike.  And it's still sore today.


My question is this -- is it possible to get a muscle spasm "down there", that will go away on its own, or is this more likely a saddle problem?  I've done long rides before without this kind of pain.