I've had that happen to me twice. With dd#2 I was with an OB who told me he suspected miscarriage because he couldn't pick up the HB in the 30 sec's of his attempted 'looking'. I had a lot of crying and then an US a few hours later that indicated everything was fine and they probably couldn't pick up the HB because baby was moving so much.
With dd#3 it happened again but with a MW this time who spent a good 5 min's looking and searching and after some nail bitting she finally found it for 1 second and then baby moved again and it was gone.
I personally wouldn't worry about it and wouldn't have an US unless they couldn't find the HB at your next appointment as well as this past one. I know it's hard but there probably really is nothing to worry about.