No worries at all! If the baby was behind the bone, it was just harder for the sound waves from the doppler to find it. Once your uterus grows more, and is mostly above the pubic bone (right now, it's probably about half and half), you shouldn't have that problem. It has nothing at all with how strong your baby's heart is-- if the u/s showed it beating, you're good to go.
It also has nothing to do with being breech. And time goes on, you'll begin to feel your baby moving around and doing somersaults! Birth position isn't determined until much later in pregnancy-- even babies that are breach as late as 38 weeks or so sometimes turn.
Congratulations on seeing your healthy baby!