Thanks for your query.
I will go with your doctor and suggest you to wait for a week and repeat a ultrasound
5-7 days after periods.
The reason is there are many a times simple cyst
which are seen before the periods and once the periods are over they resolve on their own.
For such cysts no treatment is needed.
I understand your fear as you have suffered a haemorrhagic cyst but it won't be the same always.
Please do not worry,wait for some time.
Hope this answer is helpful to you.