What Causes Palpitations And Dizziness?

I am in relatively good health (though not as physically fit as I d like to be), but over past few months have noticed my heart feeling like it is pounding with more aggressive exercise. I used to be very physically active, and am wondering if this is due to age (48) or if I should be worried about something cardiac. I am a pediatric therapist and while working with children yesterday had two episodes when bending down to pick something up that I felt quite light headed. the sensation in my chest is new and different.