is this a normal thing to have happen?

I have a quick question regarding injuries. I've been rehabbing a knee injury for a few weeks now (runner's knee...if anyone knows what that is). I've been doing uphill walking per the permission of my physical therapist instead. Overall, my knees have gotten better, but here and there I have days where they seem to hurt more than they did the day before, despite the fact that I haven't missed doing my strengthening exercises, icing, etc. Is this normal, to have days where they DON'T seem to get better?