He was the last to scrape through to the final after the semifinal where he collapsed shortly after the finish and had to be stretchered to a recovery tent.
"I have no idea what the problem was today, but my coach and I will have to evaluate that, and make sure we don't have to go through a similar type of thing again."
Willis said he went into the final a bit more aggressively than he did in the heats and semifinal.
"I wanted to be up with the leaders and suddenly found myself in the lead, it was one of the scenarios I was willing to deal with.
"I haven't had the best of success in the past leading but I wouldn't put it down to that today," said Willis.
The 2006 Commonwealth Games champion said he felt the same as he has all season.
"Never great, never bad, but when suddenly I wanted the kick I have been doing this season to follow the guys, I just went backwards."
Willis has an 800m race in Rieti, Italy, on Saturday and a 1500m in Zagreb the following Tuesday.
Meanwhile it was also disappointment for Stuart Farquhar, who failed to fire in the final of the javelin.
Farquhar was unable to produce the 80m-plus throw that had him the third-best qualifier for the final.
He opened with a throw of 78.99m and was unable to improve on that in his next two attempts and did not make the cut-off of the top eight for a further three attempts.
"It's one of those things, I had a good comp two days ago and I came to these champs feeling really good, but my timing was out - that's what happens in the javelin sometimes," said Farquhar.
Defending champion and world leader Andreas Thorkildsen of Norway was beaten by Matthias De Zordo of Germany, who won with a throw of 86.27m.
Quentin Rew finished 24th in the 50km road walk. Rew was two minutes off his personal best in finishing 27 minutes behind the winner Sergey Bakulin of Russia, in 4h 8min 46s.
