My sister had a mass/sac in her uterus, so somewhat similar. They felt that she had been pregnant with twins and lost one as she had spotting and bleeding and then what seemed like a normal period. And then carried her 3rd son to full term and now have a healthy happy super boy.
Another friend had a mass which ended up being a benign tumor that grew with the pregnancy and she looked like she was having twins ore more like triplets. The tumor was removed when she delivered. And all were safe and sound in the end.
Whatever it is, try to relax and not worry. Our bodies are miracles and heal in wonderful ways.