It is possible that the bleeding was caused by the cyst rupturing or degenerating.
I'm assuming that you went to the ER for pain, and that could be caused by the cyst rupturing or twisting. If so, when you see your gynecologist next, the ultrasound will be clear. If not, the cyst will still be present. If it is still there, and does look like a dermoid on an ultrasound, your best bet is to have it removed (laparoscopy a possibility?)
A cyst that large will continue to twist if nothing else.