At work we have a fetal doppler. I was curious to check it out. I used it last Friday and found the heartbeat no problem. Sounded fast and strong at 144-146. Last night I decided hey let's try it again and after 10-15 mins or so of endless searching, I couldn't find it. I'm 15 weeks 4 days. I left work completely discouraged something could be wrong. At my last OB appointment 2 weeks ago, my midwife found it no problem. I don't know what to do

trying hard not to be upset. Any advice would be much appreciated
Re: Why did I do this to myself?
Wife. Mom. Doula. Photographer.
BFP #1- 12/26/2011- DS Born 9/7/12
BFP#2- 10/16/2014- DD Born 7/2/15
SURPRISE! BFP#3- 11/29/16- EDD 8/6/17
DS2: EDD- 09.08.17