That is perfectly fine. As long as the heartbeat is between 120-160 then you are in the correct range. Remember that babies move just like people so when its lower its probably because your baby is resting or sleeping and when its higher its because its moving around. :)
I had one done at 14 weeks and it was 135-140 bpm. My midwife said that was completely normal.
How far along are you? I had two done, one at 8 weeks 166.67 and one at 12 weeks which was exactly the same 166.67. I think they say that 120 - 170 bpm is healthy.