Of what was the Prophet complaining during his final illness?

The signs of the Prophet's (peace be upon him) illness that led to his death were severe headache and fever. This is indicated by traditions such as Aisha's (may Allah be pleased with her) saying, "O my head!" and his saying to her, "Rather, it is I, O Aisha, O my head!" and the hadith of Abu Sa'id Al-Khudri, in which he described the severity of the Prophet's (peace be upon him) fever, and the Prophet's saying, "Indeed, we are afflicted with severe trials, and our reward is multiplied."