every now and then (about once every few days) my heartbeat will get rapid all of a sudden, and then it will do this one weird slow beat, that lasts between a second and two, and i feels like a thunk against my chest, and right as this happens, the muscles all the way up my esophagus, to the back of my mouth constrict. my heart feels like it s being squeezed, and then everything just relaxes, and i expereience a shortness of breath as this happens. i know that a panic attack is a possible explanation, but i have no reason to panic. i can be perfectly happy or relaxed, and then it comes out of nowhere. anyother explanations?