I am getting the back of the knees thing as well Dee, but not the joint pain.
Can you ring your midwife tomorrow and talkto them about it, they might have a suggestion that could help you in the meantime.
Mine said that I could use epom salts in the bath (1/3 cup at most) which would help, or lectric soda crystals (in the laundry aisle at the supermarket) - you leave them in the packet unopened and use as a compress to reduce the fluid retention.
But call them anyway to be safe, you neer know they may aske you to come in.