Need eval. If the pain is in the artificial knee it has to be evaluated for loosening, infection, instability, possible knee cap malfunction, or very rarely metal allergy. If the pain is along the inside and just over the top of the shin bone, it may be pes anseurene bursitis. Only in this condition injection may be warranted. You need to be evaluated by your orthopaedic surgeon.
No. Your knee is artificial and is unlikely to reapond to a cortisone shot.
No. Severe pain 2 years after total knee replacement is rare. Cortisone injections are helpful in the native knee and are almost always contraindicated in knee replacements. There are dozens of potential causes of your pain. Sometimes it is obvious why there is pain and other times it is extremely difficult. Consult your surgeon, or seek a second opinion regarding the source of the pain.