NST?!?

Anyone who does NSTS, can you help me out? I got them with my first but with an old fashioned machine so I saw nothing number wise. For this pregnancy I had my first NST  on Tuesday and they told me my baseline heart rate for my boy was 150 with peaks up to 180 ish. Which makes sense seeing as his heart rate is usually 154 at my appointments. I went in today and he was averaging a heart rate of 120 with peaks barely making it to 150. It even dropped below 100 a few times. I was concerned but they just smiled and said he was great and send me on my way. Is this normal? He's plenty active but I thought the drop was a little strange. Now I'm freaking out!