Why would my total knee replacement from 8 years ago start hurting now ?

Several possibilitie: These are the things that can all of a sudden cause pain at any time with a total knee of any age: tendinitis, bursitis, stress fracture, contusion, muscle strain or pull, bacterial infection, viral infection, medication reaction, soft tissue pinched between the implants, loosening of the implant from the bone cement, wear down of the bearing causing inflammation reaction to plastic particles.