I have opposite knee pain after having meniscus removal and patella surgery in other knee. why?

Common: Surgery on one knee can cause a change in leg length changing your gait or can simply alter gait by changing knee function.The 'good' knee often takes the brunt of these changes & starts to hurt. A good physician might be able to determine the exact dysfunction (ex. short leg syndrome;nerve inflammation) & provide a heel lift, osteopathic manipulation, neuroprolotherapy or other recommendation

See Surgeon: Because your gait has changed the stresses on your other knee have also changed. Talk to your Orthopedic Surgeon. Physical Therapy may be in order.