Why was Paul's body suffering while the Gentiles he reached were coming into the gospel?

Paul's body suffered because he spent his physical life and energies bringing the gospel to pagan Gentiles, who came out of spiritual darkness into the joy and life of the gospel. He endured pain, imprisonment in Rome, and hardship for the sake of the gospel, suffering the consequences of his sacrificial labor.

Les explains that Paul's bodily suffering was the direct consequence of spending his physical life and energies to get the gospel to the pagan Gentiles. While they were coming out of pagan darkness and spiritual darkness into the joy and life of the gospel, his body was suffering the consequences. Les pictures the old fellow as wracked with pain in a Roman prison, waiting for the executioner's sword, because he had been through so much all for the sake of the gospel. Verse 13 adds that he acted in the same spirit of faith, and it says he believed.

2 Corinthians 4:12

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

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.