Heartbeat rapidly changing on digital doppler at 12 weeks pregnant? Any ideas?

I am 12 weeks pregnant and have been trying to hear fetus heartbeat on digital doppler. Have been getting a reading but will flicker from e.g. 173 to 218 within a second, then stay at 218 for a second, returning to 135 then disappearing. Why is this happening? The unit will also read my heartbeat at say 68 then quickly flicker to 168 for a second. Is it picking up mine and the baby's heartbeat when it goes up to 218 or so? Could it be twins? Is it because it is so early? Please, we've already had 1 miscarriage and are very worried. Help.