Heart pounds when trying to go to sleep

Hey guys. Got diagnosed with hypothyroidism back in late July and got immediately put on 50mcg of Levothyroxine. My initial TSH was a 5.9. No antibodies, so not Hashimotos.

Now on 50mcg, my TSH is down to 3.76 and my T4 is 1.40. Took about 2 awful months but I feel a little better. However, going to sleep lately has been troublesome. When I go to sleep, my heart just POUNDS. The heart beats feel kinda heavy. Feels like it's beating incredibly fast, but checked my watch and my heart beat was only 57-64bpm). Breathing was even a little hard too, honestly.

Has anyone else had a problem similar to this? Not been sleeping well the past week or two. Would love to know what's happening? Feels like I'm having a heart attack.