why do you think emilia immediatedly assumes that othello is jealous

 Desdemona does not like Othello's tone; he seems obsessed with this object, the handkerchief,  and Desdemona is so frightened by him that she wishes she had nothing to do with it. She interrupts Othello's inquiry by bringing up Cassio's attempt to get back into Othello's favor; Othello becomes angry, and storms out. Desdemona and Emilia both note that Othello is much changed; he is unkind and seems jealous, and they are suspicious of the change in him.