knee swells slightly

I had a total knee two years ago, Recently I had a small overuse injury , that is taking forever to heal. I tried physical therapy and its been two months now and when I gradually return to activity it reaches a certain point and then knee sweells with fluid, just a little bit enough to annoy. My doctor says thisis normal but I never had this before.They said it will eventually go away. when?What can I expect. I dont overdo things at all now. I used to walk two miles a day now can go twice in half mile increments at best. Am I doomed?