Can complex fluid filled ovarian cysts become cancerous? Why might there be a bright ring around it on u/s then no ring on f/u u/s? Causing much pain.

Yes it can. Complex cysts can be cancerous depending on their size and rate of growth. A CA125 test can help decide its nature. Ultrasound is a good test to measure increase in size and also may show any solid components inside the cyst. You should get the ultrasound repeated in 3-4 months and have your gynecologist guide you further. You may need surgical removal if it is causing you much pain or increased s.