Mono/mono or mono/di. Scared momma

Tori • Been with my husband for 8 years. We have an amazing 6 year old daughter and twin 3 year old boys.
Had my first ultrasound yesterday at 7 weeks and found out we are having twins. Dr said they are measuring 6weeks and 5 days however he stated based on this ultrasound they were mono/mono twins due to not being able to see a membrane. However there were two yolk sacs seen. Trying not to get stressed but anyone have any one have any similar stories?