What Causes Heart Palpitation, Dizziness And Lightheadedness?

reacently when I m walking through the halls at school my heart starts racing thus making me feel dizzy and out of breath very fast. once I get to class I have to calm myself down in order to not feel dizzy and such. even when I m sitting during class, when I get up and walk across the room I get very light headed(it mostly happens when I m nervous)