Heart Flutter, Contraction, Quiver.?

Hello. I'm a female, 27 years old, 5'7", 160 lbs and considered healthy. I've been taking beta blockers for 6 years for chest pains and hypertension. A few weeks ago, I was lying in bed reading when I felt like my heart shrunk. Right after that, it started beating hard and irregularly; it really scared me. I thought it was a heart attack and that I'd soon be feeling pain, so my first instinct was to breathe deeply, but then it all went back to normal.

Today, I've been having flutters in my heart all day long. This happens rarely since I've been taking my medicine. But my question is, is this normal? It's right underneath my left ribs, toward the bottom of the breastbone. The sensation is like a muscle spasm or your heart literally quivering. I can feel the flutters on different sides. Then sometimes the flutter turns into a strong heartbeat.

I had a cardiac stress test 2 years ago after my doctor thought it could be mitral valve prolapse, but everything seemed okay. My doctor says these flutters could be due to stress. Has anyone experienced this? Should I be worried?
Come on! No opinions?