Is It A Matter Of Concern If Having Heart Racing After Exertion Considering Having Had Mitral Valve Replacement And Afib?

I was operated when I was 5 and had my mitral heart valve replaced with an artificial heart valve and have been on warfarin ever since then. I was diagnosed with afib 2 years ago and I was on and off beta blockers "metoprolol" since then, i started experiencing weakness, random chest pain, nausea, anxiety and depersonalization i recently got a check up jan 14 to be exact and the doctor told me to come off of the medicine and made it a "pill-in-a-pocket" and only to take it when I need it. however now it seems like i experience sort of like an acute sinus tachycardia whenever I eat a big meal my heart would race to the 80s-90s to the low 100s and slowly trickle down as time goes on the thing is I also experience GARD symptoms after I eat and I'm not sure if that's related. my resting heart rate usually varies from 60-68 bpm avg..however I also get a heart palpitations from little exertion when I attempt to work out or doing anything that requires a little more exertion my heart rate would shoot up fast, I use to work out regularly and that never happened before could this be a concern? I went to the ER twice prior to the check up and they said it might be stress related but I want to make sure. Could this be a concern?