What Causes Palpitations And Weakness In Limbs During An Activity?

I am a fairly slender, fit woman of 65, but I frequently--numerous times a day--get these sudden weak spells where my arm and leg muscles feel like they are melting and my heart thuds a bit. I only have to slow down what I am doing (climbing stairs, jogging) and the symptoms will go away in a minute or so. But this is becoming so frequent that it is getting bothersome. Any thoughts? I take no prescription meds.