Before we look at Paul, it is important to understand that Jesus warned us that there would be lies in the Bible:
Jesus warned a deceiver would come after him. Paul (who came after) outright admitted that he (Paul) was a false disciple and a liar and warned that those who follow him are fools because he does not speak after the Lord, but foolishly.
Why did Paul speak so harshly of himself? Most likely because he was not a willing participant, but under threat of death if he did not write what Rome wanted him to write. The contradictions in his letters prove he was telling the truth: His words did not come from God: