While she is on the roost tonight you could catch her sleeping, and feel of the bubble to see if it is hollow with air or fluid. I don’t know if it would be the correct thing to do, but I would be tempted to clean it with betadine and lance it, to see if it deflates. A vet of course is always best.
Usually these types of blisters resolve themselves and get absorbed by the chicken's system. But this seems to have lasted beyond that. I would lance this bubble, whatever it may be.
Wipe the area around the bubble with alcohol or Vetericyn or Betadine. Insert a sterile needle at the base of the bubble where it meets the facial tissue. Report to us what comes dribbling out. If it's just clear water. Press the remainder of the fluid out and spray with Vetericyn or wipe again with Betadine.
If nothing comes out on its own. Put a slight amount of pressure on the edge of the bubble to encourage a drop of the contents to emerge. If it's a waxy substance, it's likely infection. If the contents are putrid smelling, it's almost definitely infection.
If nothing comes out, it's logical to assume it's a tumor or cyst.