I haven't heard abt the ill effects of doppler but have heard that babies don't like them much.. so try to move away from the doppler.
Hide and seek pretty much explains it..:)
I have had my doppler for about 3 weeks now. I have used it many times. It is getting easier to find the HB now. The earlier you are the harder to find, if at all. During week 11, I had to press down and up into my pubic bone to find the HB, it is starting to move up gradually now though. I have noticed that sometimes it is harder to find than others. If baby has their back to doppler, I read that it is harder to find. I find that my little bean is getting really active and just when I find the heartbeat I hear a loud swoosh and it is gone. I figure he's playing hide and seek.
I have used mine a lot since getting it, and I love it. I have read a lot and havent found anything to say that prolonged use can cause damage. If you are sore, it is probably just from the extended pressure. How far along are you??
If you are nervous, or feeling unusual pain, you need to see your doctor, just to be safe.