What Causes Knee Pain After Ankle Surgery?

i had ankle surgery 3 months ago and day by day it is getting better. now i had been advised by the doctor to walk by putting first 25% load for first two weeks and then 70% load which is over now. Now i am walking without any support for the last five six days but after few days i am having pain in my knee and i want to know is it because i was not putting much weight on it and now it is in use. I think it is because of inflammation and is it ok to take chymoral forte?