I have a fetal doppler and i'm just reading on how safe they are n i've came across info that they cause heating when held still and brain damage?

No: That is completely false. You may have read that there are ultrasounds that are high frequency ultrasounds (machines, generally) that can generate heat, but those aren't the type that are used for fetal doppler. There have been a lot of studies on the safety of ultrasound in pregnancy and there's no literature or data to support any harm to the baby. Not to worry.