Heart beating harder than usual and there is a slight pain in the middle of my chest. what could be wrong?

Possibly nothing : The sensation could be many things, including extra heart beats (not typically dangerous) , an arrhythmia (abnormal heart beat), anxiety, or dangerous conditions that cause chest pain like a heart attack or aortic dissection (both much less likely). Try taking your pulse in your neck or wrist while you're having the sensation -- if it's faster than 100-120 /min, go have an ekg done.