23 male, 5ft 11, 140 lbs

I've been anxious over my heartbeat despite having an ecg done and the dr saying its good. Sometimes I notice my heart beats like hard and slowish while other time it feels like it beats really soft but kinda fast. is this a normal thing hearts do?

Is that just normal  heart variation? around night time its slow, like high 50s low 60s and either very quiet or a bit hard. usually in the morning I get this almost "flutter" where its like weak but kinda fast like 100-120. Throughout the day its more steady at like 70-90. 

Is this normal?