Doppler Problems/Scare

So I'm a little worried... Around 9 weeks I bought my own Doppler and was able to find the baby's heart beat rather easily. I am now 11 weeks today and for the last two days have not been able to find a heart beat. I haven't had any spotting or cramping yet. But I am worried something is wrong. I know the baby may have just moved or changed positions (I'm hoping), but I'm still worried. Has anyone else experienced this... where they were able to find a heart beat and then later they were not able to? Thanks