When did you first hear your baby's heartbeat?

My last prenatal appointment was at 13 weeks. The nurse told me that I probably would not be able to hear my baby's heartbeat yet but she would try and work a miracle. Well after an attempt that seemed like an eternity, the nurse was unsuccessful. She told me that the doppler detected a heartbeat on the monitor but it could not be heard. She said I should definitely hear a heartbeat at 16 weeks. Has anyone else had this experience. I am very worried. At 8 weeks I had an ultrasound and the doctor said my baby had a very strong heartbeat. Why couldn''t I hear it. I am very worried. I have a 3D ultrasound scheduled for thursday, I will be 15 weeks.