Worried about pain in my lower abdomen?

Actually hearing the HB with doppler is a pretty big milestone! The baby may not even be in the same place now. It sounds like stretching of the round ligaments of the uterus--very common (and since it is caused by increases in hormones, it's also a good sign.) I didn't have this with my first pregnancy either. I would go about my normal activities. if i were you. Take a deep breath, your baby is likely doing great!!