It was a bit naughty of your midwife to look for a HB that early, she must have been a bit over confident with her dopplering skills!
Standard time for a midwife to use a doppler on you is 16 weeks, because its so hard to find before that and can take so long to pinpoint it would triple the length of appointment times!
I have heard my baby's HB since 9w5d, but thats because my doppler has a probe and I listened to it through my twinkle

I won't tell you not to worry because I know from experience that you can't just press a button that switches off worry, but do try to remain calm as most likely you just have a little ninja in there that was hiding! They can hide pretty effectively, I'm almost 18 weeks and the other day used my doppler because I had not noticed baby move that day and it took almost 20 mins to find! Highly unusual for me...but proving that babies are so good at hiding from dopplers they can do it even when they are quite big and I am very good at using my doppler, it shocked me it can still take so long to find at this point!