Two Heartbeats, One Baby?

I just would like opinions on this situation.

I am about to be 17 weeks, every doctor appointment has only had one heartbeat and the only ultrasound picture, attached, shows what looks like one baby.

But my boyfriend and I have a at home doppler and he have found 2 different heartbeats beyond mine. One gets up to 170 and one no more than 140.

This pregnancy is a lot rougher than my first with my son. I have to take pills for morning sickness, I have had a headache for over 2 weeks now and I am way more exhausted. The only thing making me not think twins is that I am not very big, but I wasn't big with my son either.

What are you're opinions on me having twins or just one?