It certainly sounds like a Baker'sCyst.
Let me sum up for you......you partner is suffering extreme pain from an undiagnosed mass behind her knee, which cannot be controlled by the appropriate analgesics and you have come on AB for comments?
"Medics are good at telling you what "it isn't" Has anybody out there suffered similar?"
It is often very important to tell patients "what it isn't" just as important as telling them what it is.
If the situation is as you describe, then see you GP or A&E dept.