What could be wrong with me?

Sometimes during the week I start to feel faint and it almost feels like my blood pressure drops. The veins sink in my hands/arms and my breathing becomes shallow and deep. I check my pulse and although my heart rate stays constant, the strength of the beating against my fingers feels weaker as if my blood pressure has dropped low. This happens once in a while, maybe about twice a week and if I can control my breathing or having something to eat or have a gl of water I can immediately feel my pulse become stronger again. I'm 16 years old and I was just checked by a pediatric cardiologist last week as part of a check up and was fine. What is this?