Swollen knee. No pain/injury. Already went to Ortho.?

OK well this is a pretty long incident and i'm sorry but i really need some help. Well, one night i noticed i couldn't bend my knee. I took a look and found it was HUGE. I mean real big. I hadn't played any sports for a long time or nothing, no injury or no fall. but three day's before i had walked about a mile home but the doctor said that couldn't have caused it. We Went to Urgent Care and they took one look at it and sent me to the Emergency Room. At the emergency room, the took out the fluid, sent it to get tested and took some X-ray's. After waiting some time, they said i was OK to go because they got a call saying there was nothing strange in the fluid so it wasn't infecter or nothing. They gave me some crutches a knee immobilizer and we set up an appointment with an orthopedic.They also prescribed some pain killers and for me to elevate and ice me (which i did). We went to the orth. a week later and took the x-rays and he reviewd them and said everything looked great! He also said he never got the results of the fluids so he had to take some out too. He then gave me some Naproxen and i took one every morning and every night with food and a whole gl of watet. Then we set up an appointment for an MRI. When i got it, We went back with the same ortho. and he reviewd them. Everything also looked great! At this time, the swelling had gone down by alot. he told me he couldn't find anything wrong & he couldn't help me anymore because i didn't need any surgery or anything. then he said to get off the naproxen and start bending my knee more because it was getting real stiff but still continue to ice it. Everything seem great for about a week but yesterday my started to swell up again! The times before, it was soft and kind of squishy (i'm guessing because of the fluid) but now it's really, really hard. What can be causing this? What should i do know that the doctor couldn't even help?