Hi tigger bounce,
I have a dopppler and found heartbeat at 9 weeks - and i'm a size 16 but do have a high uterus already. I would say that its sounds as if you have found your own heart beat! You can find it loads of places down there! along with whoosing and other noises. Babies heartbeat should be between 120 - 180 about. The easiest way to check if its your own is to feel your pulse at the same time and see if its the same. I made the same mistake as you and was convinced it was the babies and then worried that it was to low until i did some research and found it was mine!
A few tips to finding it: it takes time! You need to move the doppler very slowly, starting at the pubic bone and in the centre, you wil need to place the doppler with plenty of gel and before moving it to either side check all angles with the doppler - again very slowly. A full bladder is a must as it pushes your uterus up. You also need a quite room and need to lie as flat as possible. The only other piece of advice i can add is to say that it takes time, when i finally found my bubbas i was determined and spent over half an hour looking, meticulously checking every area possible.
You will know when you hear it as it is twice as fast as your own and sounds completely different to you own - it has been described as galloping horses. It took me a week or so to get the hang of mine and i can now find the heartbeat in about 5-10 mins as i know where to look. In fact i can find it on both sides and am convinced its twins, but it could be one big one lol.
Let us know how you get on, i hope the advice helps.