I got really scared and my heart started beating really fast. I got shaky then really weak. What's wrong? There was nothing there.

Order... If the emotion clearly precedes the rhythm, and the rhythm is something called sinus tachycardia, it's generally an anxiety or panic issue. If the fast rhythm started before any anxiety or emotional state, a rhythm is frequently to blame. An electrophysiologist is a type of cardiologist who can help.