I thought of really bad indigestion and following Soliaree's post where it was her gall bladder, it could have been something like that. I know I get really uncomfortable and achey and now know to take a Pepcid Complete and that helps (and often this happens when I haven't eaten or done anything).

I had problems with my arm/shoulder all summer and it took 3 months to get that to go away; you could have had some sort of nerve "twist" or something that straightened itself out.

You didn't imagine it or have a heart attack so I would just chalk it up to "one of those things". I use to get weird things like that (once got a fever of 104 out of the blue when I was about 26-27, with no other symptoms (besides the tiredness/weariness that would naturally go with such a fever), no aches or pains, no headache even, and it was gone within a few hours.