Use of fetal doppler 10 times a day, Safe or not?

I went to the ER since I had breathing difficulty which was caused due to Bronchial Asthma. They hospitalized me and kept me under observation. In 2 days, they've used the doppler on me some 20 times, around 5 mins every time and once for 40 mins for some NST. I am now worried at the frequency of their usage. The nurses keep doing this but I haven't checked with my Doctor yet. What do you think? Why do they have to check when I am monitored and everything in me is fine - my BP, sugar, urine, temperature etc.