Atrial fibrillation. Quite common among older endurance athletes. I know at least two with this problem. Exhibits as very rapid HRs, 140-170 at rest not uncommon. Accompanied by feeling of weakness, lightheadedness. I don't know of an instance of it being diagnosed when it was not actually occurring. Doc usually attaches a Halter recording device to the patient for 24 hours, instructing them to attempt to repeat whatever it was they did that prompted them to come in. I had that done for a different problem, a heart that would just stop beating for a bit while resting. The device is not uncomfortable. If you experience it again, do call 911 and maybe the medics can get a recording. This link might be edifying: http://europace.oxfordjournals.org/content/6/3/222.long

And teh google is always informative.