Nah I dont think you have anything to worry about either. Its the same with ultrasound scans. Even at 15 weeks they were pressing right down in my pelvis region to find the baby. I think it probably all depends on how your little one is laying.
My MW said to me at my last appointment "Ok, found it perfectly on that side, now lets check the other to make sure theres not two in there", and funnily enough, it was beating on the other side aswell, it was very strange.
So again, I think the low/high all depends on how the baby is laying at that point in time.
I have the monitor that straps to your tummy, and even now I have to put it fairly low to find Nick Nacks HB and I can feel him kicking around in my ribs now-so I'd put it down to babies position, So I wouldnt worry! x