I live in Los Angeles, and don't have the variations in temperatures, humidity, and barometric (sp?) pressure that people living in other locations might. But I do have good days and not so good days. My knee is stiffer when I haven't been up and moving about. And if I overwork it, it lets me know how unhappy it is -- big time. I think that's all part of the recovery journey, as well as our body's way of giving us feedback on how we're treating our new knees.
Does your knee look larger? Is it warm or hot to the touch? Are you increasing the time on the exercise bike per your OS' plan? Have you increased the resistance? It could be that you're overworking the knee and it's letting you know. Try easing off on the PT for a day or so and see if that helps the knee settle down.
Speaking of rain and humidity, etc., mind sending some of that over to us in drought-weary California. No fair that you're keeping all that water to yourselves! LOL