Sonoline Fetal Doppler

Raven • 21, happily engaged and expecting our little rainbow child!
Whelp, my fiancé bought me a fetal doppler to calm my nerves and here I am stressing out more than ever. I've used it once, the heartbeat was lower than it was yesterday (at my appointment) and I'm thinking it was because of the different device? It was also louder at my appointment. 
And I'm such a ridiculous worry wart that I'm scared the waves of a cheaper device are going to hurt my baby. I'm such a nut when it comes to these things, but I'm so worried about losing this child! 
Can someone calm my nerves about having used this product?