from the brief history u have mentioned I presume u must be a individual with higher body weight and u have skipped the medication for more than 5- 7 days of at all u feel that stopping the medication has caused it.
then again the heartbeat which u have mentioned is called palpitations and in ur case it should be because of an increase in end diastolic volume due to overweight
and reduction in heart rate
due to hypothyroid state as a form of compensation by the body.
if I have more details probably I could help u out more.
feel free to contact