Heart thumping out of chest

My heart started pounding, thumping and beating sooo fast for about 5 minutes out of no where. Beating so hard my whole chest was vibrating.. this is the second time this has happened to me.. first time was back in January, same thing out of no where while I was doing nothing, hearts pounding like crazy and beating so fast for a few minutes, I really thought I was about to go into cardiac arrest or something.. just happened again and lasted about 5 minutes, very scary and uncomfortable feeling.. should I be concerned? Is this common with graves? Obviously I already have a high resting heart rate but it never pounds or beats that hard!! I do have beta blockers but I stopped taking them months ago because I felt like I didn’t really need them. I feel like if I bring this up to my endo all she will say is “well that’s why you have beta blockers” lol… anyone else experience this?