It was a wonderful, wonderful high. He was feeling real good after it. But in the Christian life, we are to be dependent upon God at all times. And so, God, not wanting Paul to be too elated, not wanting Paul to be floating around on Cloud Nine all the time, but instead wanting him to be firmly planted in reality, permits what Paul calls a thorn in the flesh, a messenger from Satan. What it is we don't know.
Biblical scholars love speculating on it.
It's something physical, something mental, something sexual. It's the persecution he must undergo. Paul doesn't tell us what it is and we can only guess. The purpose of the thorn, though, is clear. It is to keep Paul from being too puffed up with pride.