Is it normal

Might be a silly question, My baby heartbeat at my 16wk ultrasound was 142 but at my 20wk+2 his heartbeat was 120 but the following day at my gender ultrasound his heartbeat 163 is it normal that it is so dramatic In change? I had to have a stat blood draw because the Dr I went to for my gender ultrasound (Maternal fetal medicine) said it was a little worry some his heartbeat seemed irregular. Has anyone had this problem & everything was fine?