I don't understand what is going on

Joy • My baby is coming January 3,2016
19 weeks and 4 days today and Ftm,this morning I start spotting,was light pink,I went to see the doctor and did the us,baby was fine and the hart beat was strong,is the second time that is happening ,I'm so stressed because we don't know why is happening ..have any of you lady's have the same problem and had a healthy baby?