Hello, I would be happy to help you with your question.
First of all, there are two possibilities:
1. You had a twin pregnancy
and the second twin miscarried very early and the sac is being reabsorbed
2. You have a "subchorionic hemorrhage" which is bleeding under the sac. This is very common and most often it is of no clinical significance.
In answer to your questions. Yes twins usually develop at the same rate. At about 6 weeks, if you had twins, they would have seen both sacs, both yolk sacs, and likely both heart beats.
I hope things are resolved and everything goes well! Good luck!