the exact same thing happened to me, i was having some spotting/bleeding, so i went to the ER, there they took some blood samples and i had an ultrasound, at 5w6d, they looked for the fetal pole and sac, which were there, but there was no heartbeat showing up, so they said it was probably to early to catch it on the monitor, so they had me come back a week later, and so at 6w4 days i went to get my followup u/s, they did detect a heartbeat, at 100 bpm, and they said (those people at the ER) that with my bleeding, and the baby's heartbeat, that i was most likely having a slow miscariage... and should be monitored more closely, so of course, after they told me not to panic O_o we went home and tried not to panic (yeah really)
so all this to say that i made an appointment with my gynecologist after seeing the ER doctors , i've seen her twice, once at 8 weeks and again yesterday, at 10w, 6d, and she explained to us that there was absolutely nothing wrong with the baby, and my husband asked if there would be problems with the baby since the heartbeat was so slow at 6 weeks, and she said, (and she even had another gynecologist come in so she could confirm this information) that, early in the pregnancy it is normal that the heartbeat would be detected as such, since it's very faint, the monitors will not be able to accurately catch the heartbeat,(in my case the ER doctor could only hear it with the transv____al ultrasound wand, and not at all with the transducer - at 6w4d)
so today i am at 11 weeks, our obgyn showed us on the u/s the baby's heart beating, explained the rate of bpm, and says that it is normal of course that we were worried, since the ER doctor told us the baby was practically in distress, but everything is proceeding, in our case, quite normally, right size and weight AND heartbeat, i would not worry, it's not good for you nor the baby, both obgyns i saw said that it is normal that so early in the pregnancy it would be harder to catch an accurate heartbeat, but that doesn't mean that the baby is in any danger or distress, it's just too faint to catch on monitors
i would reschedule another appointment after 8 weeks to check again and to rea__sure yourself that the baby is doing fine, i was going crazy just thinking about it.... not knowing is the worst i know :(
i hope you have a wonderful healthy pregnancy, and hopefully this helps, next time you have an ultrasound, ask as many questions as you can, ask that they explain everything that is going on on the screen, you will see it greatly helps, good luck!