Bianca suspects Cassio of being inconstant in his affections towards her. Let us remember that Cassio has given her as a token of his affections the handkerchief that belongs to Desdemona, though he does not know this. Bianca suspects that Cassio has therefore deliberately insulted her by giving her as a token of love and esteem the token that he has received from another woman. Note what she accuses him of as she storms in:
Let the devil and his dam haunt you! What did you
mean by that same handkerchief you gave me even now?
I was a fine fool to take it. I must take out the
work?--A likely piece of work, that you should find
it in your chamber, and not know who left it there!
This is some minx's token, and I must take out the
work? There; give it your hobby-horse: wheresoever
you had it, I'll take out no work on't.
From Bianca's perspective, the handkerchief belongs to "some minx" and was given to Cassio. She sees it as proof that Cassio is having other affairs and relationships at the same time as courting her. In addition, it is clear that Cassio asked her to unstitch the embroidery that was done on the handkerchief so as to make it unidentifiable by the real owner. This is just another sign of how inconstant he has been from Bianca's perspective.