Where was Paul and in what condition when he wrote about his sufferings?

He was in prison in Rome, awaiting the executioner's sword, wracked with pain. His body suffered the consequences of spending his physical life and energies to get the gospel to the pagans, and he had been through so much for the sake of the gospel.

Les explains that as a result of Paul spending his physical life and energies to get the gospel to these pagans, they were coming out of pagan and spiritual darkness into the joy and life of the gospel, but his body was suffering the consequences. Les suggests that the old fellow must have been wracked with pain as he sat in prison in Rome waiting for the executioner's sword, because he had been through so much all for the sake of the gospel.

2 Corinthians 4:12

So then death worketh in us, but life in you.

Why was Paul's body suffering while the people he preached to were being saved?

Paul's body suffered because he expended his physical life and energies to get the gospel to pagan people, who were coming out of spiritual darkness into the joy and life of the gospel. His bodily suffering was the consequence of that labor, and he endured pain in prison for the gospel's sake.