"Nobody knows", is the simple answer, because the Bible does not state what it was. Church stories / traditions may offer some word-sketches as to Paul's physical appearance and health, but none of them can cite anything the apostle Paul wrote outside of the Bible where he might have specified it.
Indeed, Paul seems to have been careful to avoid saying what, exactly, it was. Even in his letter, 1 Corinthians 12:7-9, he is non-specific mentioning an illness described as "a thorn in my flesh". He said it was from a messenger of Satan, to torment him, and despite asking God three times to take it away, God ended the matter by saying to Paul, "My grace is all you need. My power works best in weakness.." (NLT).
It was taken as understood that folks in the congregations he personally visited would know, and that it was not necessary for anyone else to be informed in writing. A foot-note in the NIV Study Bible says re. Gal.4:13:
"13 As you know, it was because of an illness that I first preached
the gospel to you" - illness. On the basis of v. 15; 6:11 some suggest
it was eye trouble. Others have suggested malaria or epilepsy."
Perhaps the biggest clue is in verse 15 where Paul adds, "I can testify that, if you could have done so, you would have torn out your eyes and given them to me." The eye-problem idea has a lot going for it, I suppose, but even if it was that, there's a whole array of eye diseases that it could have been, and we just do not know.