two gestational sacs, one heartbeat

Sarah • TTC since April 2016. MMC #1 Sept `16 MMC #2 Jan `17 Ectopic and Tube Removed Nov ‘17
This may not be the right group, but don't know where to turn! Has anyone experienced this?
One sac has little baby in strong heartbeat but a bit smaller than worked out, the other sac measuring about 3 times bigger than the baby one and is evidently empty. They think it might be a twin that's too small to see, a twin that's failed to thrive or just an empty sac. 
I'm scared it's going to push baby out :( and I've got a bicornuate uterus as well, and both these sacs are in one side so a bit overcrowded! 
Can anyone please give me any information? I'm scared this is going to end in another miscarriage :(