Othello is an insecure individual; this is as a result of his difference with the rest of his world, the white-world. Othello being black, has given him a sense of instability where he is living a "dream" and this dream could come to taters at any point in time, whence he will "be put back in his place" with the lower class. Hence his cautious and vigilant essence is really what made him and what continues to make him what he is.
Act 3 Scene 3 line 265 "haply for I am black and, and have not those soft parts of conversation that chamberers have ." .
Othello believes that he has done everything correctly till that point in time and consequently takes pride in this. His reputation is pristine, and as a result is well respected; however this breaking news cunningly delivered by Iago brings him to pieces as he begins to delve within himself with the repeated teasing questions of "how?" and "why?" This is also in support of the judgement that Othello is an anxious and insecure individual. With no answers he falls apart and into this epileptic fit.
His disbelief causes Othello to question himself Act 4 Scene 1 line 40 "Pish! noses ears and lips". Is't possible? - confess? Handkerchief? O" Devil!".
"The Great Othello" that had once sat on a high stool and was deeply feared was no more. Desdemona's "unfaithfulness" had created and ignited a fire within Othello, a fire that told him this occurrence would threaten Othello's high standing. Iago had plotted this very shrewdly and recklessly he knew that the only way around a man like Othello would be through him, and that's exactly how he played it; he mixed around with the moor's emotions and planted a seed that evolved into a deadly weed. It was the thought in Othello that Desdemona would prefer a man other than him, and do it so openly with no fear of the outcome that terrified him the most. The fire inside beckoned him to put an end to this evolving threat.