Thank you for your question. It sounds that either you have a nerve compression or nerve injury after the surgery, although without being able to examine you difficult to say anything. Additionally, it is essential that you gradually adopt the physical strength and avoid streneous activities. For precise examination please consult your treating doctor. Hope this helps.
Have you told your orthopedic surgeon who preformed the replacement about this?
How does it radiate from where to where?
Does it start directly behind the knee and travel down? Have you noticed any increased swelling?
I'd be contacting my doctor, could also be indicator of the beginning of a blood clot,(the popliteal artery is found behind the knee, a common place for clots to form) the knee and calf area especially since you've stated that you experience this both at rest as well as with exercise.
I personally would either a) contact my PCP or b) contact the surgeon who preformed the surgery for an appointment or c) go to the ER's express care service with the complaint.
A blood clot isn't anything to mess with, it's better to be safe that sorry.
Good Luck. Hope I didn't scare you. Just according to what you did say, this is my first impression, especially since you are only 8 weeks post surgical.
Purchase "Stop Pain" spray. You will spray the area that hurts and then take a warm/hot shower. You can also warm up a wet towel, heat it up in the microwave, becareful with the heat, about 30-45 sec. not too hot or use a heat pad and place over the area. This will take the eadge of the pain.
I suffer from fibromyalgia and have major injuries that cause me extreme joint pain and I do not take medicine, I just spray this and place heat and it does the trip. my best wishes to you.