What Causes Throbbing Pain In Shin?

I have a worsening pain (throbbing) in my right middle inside shin. I had a total knee replacement 5 months ago and the shin pain started after physical theraphy. My knee is also painful, but the shin is worse and bothers me mostly at night. My MD took an x-ray of the affected area, but it was negative. There is some minor swelling, but it seems to vary. Rob