Fluid/swelling can be a sign of soft tissue damage. Back in April I damaged something behind my left kneecap, and I've had some significant swelling off and on since then... but my doc checked it out and proclaimed it okay, and recommended PT to help strengthen the muscles that support my knees, which ultimately is why I started biking.

When it's swollen, my knee feels very "tight" - it's not so much that it's painful, but I definitely feel it. I take it as a sign to take it easy. Cycling doesn't bother it, but walking for any amount of time does.

When I tore my ACL in my right knee (ages ago), I battled swelling in that knee for close to 2 years before everything was solidly back to "normal", but I also had a lot of other soft tissue damage that was cleaned up in surgery, so it wasn't "just" my ACL repair that had to heal, but basically my entire joint, and anything I did irritated it, so it was a long process. Even now nearly 15 years later, occasionally I'll tweak it and get "some" swelling, but nothing significant.