Dizziness And Extra Heart Beat

Everytime I stand up from a seated postion or sit up from a lying postion I get dizzy and my blood pressure drops. I went to a cardiologist who put a 24 hour halter monitor on me and it showed when I have these episodes I have an extra heart beat. The cardiologist more or less said I was a fainting goat and he didn't want to see me again. What I want to know is, could this cause serious problems for me, should I talk to another doctor, or am I a "fainting goat"? I'm a 24 year old female. I exercise six days a week, drink 64 oz. of water a day, and I eat several small meals a day. When I exercise, my heart rate goes from 74 bpm to 225 bpm in less than five minutes, I get short of breath, and my chest and left arm begin to hurt. What's going on? Someone please shed some light on this for me.