Ultrasound lady has stressed me out!?

Had 20 week scan today and she was mentioning about home dopplers and how they have so much heat and are so bad for the baby and that's why doctors don't use them all the time..

She then told me that research shows they kill cells in the baby and can cause death!! So now I've used my doppler a lot since 13 weeks and now I'm 20 weeks sometimes once daily for a few minutes and now I'm stressing.. hope everything is okay. I did Google before using it and I couldn't find any proven facts but now I'm worried 😢