Heart rate falling!

I am 13w1d and I got a home fetal Doppler to monitor my baby cos I was so paranoid about something terrible happening all the time. I have been easily able to locate the heartbeat for the last week and it has always been between 155 - 165. Today, the max heart rate was 129, and it never went higher than that no matter how many times I tried. Should I be worried?? Is something wrong with my baby?