Have Chest Palpitation. Healthy And Follow Good Diet. What Could Be The Reason?

For the past year and a half i have suddenly been experiencing chest palpitations that sometimes result in mild cough.
i'm female, thirty years old, more active than average person, not over weight, good diet(all four food groups plus occasional pizza) sometimes drink coke but mostly decaf mint tea.
I dont feel stressed when it happens, not prone to anxiety, but its been happening every other day and usually when i'm sitting down. also sometimes feel light headed if i stand up too quickly. afraid of the answer!