I am a long distance runner. sometimes, during my runs, my heart suddenly starts to race. it beats really fast and i get dizzy, tired and nauseas.

Need cardiac eval: It sounds like you may be developing an arrhythmia or irregular heartbeat when your heart is stressed to a certain point. You need to see a cardiologist for appropriate evaluation. Do not delay with this. Sometimes these abnormal heartbeats can be fatal. Best of luck with this. Dr R

Get it evaluated: You will probably be given a Holter monitor to run with. It will determine what kind of rhythm disturbance you are having and whether I needs treatment