Fetal heartrate dropping from 150's to around 100 and back up?

ok...I have been listening to the baby on doppler lately and there are several times I have been listening that the babys heartrate will just suddenly drop from around 150 to about 100...It comes right back up but this is weird!!! I know it isnt the baby moving and the doppler dropping it cause I physically hear the drop and rise along with the doppler read out....What causes this?! I am 16 wks 4 days today....