Swelling, Pain, Weakness - any ideas?

I am suffering from severe swelling and pain in both of my knees. I have not had any injuries to either knees, and they don't normally swell at the same time. They started to swell about a month after I gave birth to my first child.
I feel tightness in the back of my knee, then my whole knee (back and front, but mainly front) starts to swell, then they become extremely painful (I think mainly due to the excessive swelling), become very weak - they lock and give way. They cause me to limp severely when swollen - I can barely bend them either when swollen. I cannot get out of a chair without pushing myself with my arms.
I have been to my family doctor (who has NO IDEA what the problem is), an ortho (who said it was not an ortho issue as the symptoms come and go and effect boths knees sporadically), and am waiting to see a rheumatologist.
My RA factor came back negative and my sedement rate was normal.
Any ideas, or anyone else suffered from the same thing? I'm getting pretty desperate as it's getting worse and previously I could go weeks without symptoms - now when the sweeling goes down in one knee the other immediately swells up.