hello, hopefully after thyroid removal you must be on oral thyroxin (drug). if not then this condition of yours can be related to decrease blood pressure which lead to reflex increase heart rate
, if you are on drugs then the doses need to be adjusted, are you having other symptoms like constipation
, intolerence to cold, tiredness
, weight gain with poor appetite
etc which again suggest above explaination . hope i satisfy your query. have a good day..