Low fetal heart rate

Is it normal for the heart rate to drop so drastically at 16 weeks? I went in last Tuesday and I was 15 weeks 4 days and her heart rate was 147 I went in again today because I just haven’t been feeling good and her heart rate was 136 is that normal? She was moving around a lot but last week she was sleeping... The doctor said the heart rate was normal but it completely slipped my mind to ask if that’s normal I just want to cry because I’m so scared something is going to happen to her