Heart pounding in chest after climbing one flight of stairs?

Tam Lou Asked: Heart pounding in chest after climbing one flight of stairs?

Hey guys, i've been having an issue with my heart and wanted some advice. When I climb the stairs at home (12 stairs) i usually run or walk quickly up but when I get to the top my heart it pounding in my chest, sometimes I think I can see it beating away, it goes up to 180, my resting heart rate is usually around 70. I'm really fit
normally, get my heart rate up and sweat a lot at the gym 6 times a week, cycle everywhere and recently ran my first marathon so I'd say I'm pretty fit. Just wondering if anyone knew what this could be? Past medical history is minimal, my blood pressure is low but not dangerously low, usually around 105/65. I eat super healthy and this only happens after climbing the stairs at a pace faster than a jog. It has never happened before or during exercise. Thanks in advance!