Dropping Progesterone

So I went today for weekly blood and scan. Saw the embryo and the heartbeat (90bpm). The doc said hcg levels and a heartbeat are good, but that the heartbeat was a little slow (I'm barely 6 weeks) and that my progesterone level dropped ( from 17 on 4/18 to 10 on 4/25). She prescribed progesterone pills that I insert vaginally. Should I be worried?  I feel like doctors can be so gloom and doom. I was so happy when I saw that heartbeat, and now I'm all worried all over again.