About 6 years ago I had a situation when i was exercising where I was having a very hard time recovering from the exercise, went to the hospital had an angioplasty, except when the got to my heart they found now unusual blockage so the didn't do the plasty part of it, it turned into just an angiogram. I also had a stress test etc and were completely unable to figure out what happened. The cardiologist thought it might be a "cramping" of the muscles around the aorta cutting off some circulation for a while, my GP who knows I have a history of arrythmia, thought I might have just gotten caught up in a particulalry bad episode of it. There was no pain at all, just felt terrible.
If you are concerned, go to the nearest Urgent Care or hospital.
Do you have a history of this type of problem? Any prior exams, tests?