Severe knees pain almost unable to move. Running above 1c temperature fever. Just had tetanus shot n a pain killer injection last evening. Why is it???

Immune reaction. I'm assuming you were well until you got your tetanus shot. Because of previous vaccines, your body has memory or antibodies against tetanus, so that when this shot was given, your body's immunity is fighting it, as if you had a new virus in your system. This can cause fever and joint symptoms and sometimes rashes. It usually resolves in a day or two. If you were ill before, contact your doc.