Can anybody explain what this is!!!?

All of a sudden last thursday i felt my heart begin beating very fast like 160bpm which only lasted about a minute or so and went back to normal suddenly. The it happened again later in the day while I was still at school. Then when I got home it felt strange like it was jumping around in my chest and I thought I was gunna die. Then since that day I haven't felt rite and my heart will skip beats when I walk around or take a shower and then if i do even minimal exercise it will beat very fast again and feel like it flopping around in my chest. I saw a cardiologist who told me it was pacs and svt and it wasn't anything serious but it just doesn't feel rite at all. I can't do any exercise or even shower without it pounding and feeling strange. I've never had any medical problems before and no heart problems whatsoever and this literally came out of the blue. Can someone please explain what this could be?