Thanks for getting in touch with us. I tnhink that you have every right to ask for further investigations here - you're in a lot of pain and now even the good knee's hurting, so it's something that obviously needs looking into. Physio may be helpful, but you also need to get the swelling to subside.
You can get your knees aspirated, where they take the fluid off them, then usually follow this up with a cortizone injection into the knee joint.
So yes, Pam, I would go back to the doctors. There's no way you should have to put up with being written off.
Let us know how you get on - we can always send you some of our written info if/when you have a definite diagnosis.