The fish are a metaphor for social anxiety
I think that was just the case for the big carp that dislocated her jaw. When she confessed, it was a different species, whose Japanese pronunciation was a pun on "kissing" (plus, the fish literally kissed as well). It seems the various fish species represent different emotional states, social anxiety being just one of them. Note that she says people started avoiding her because of the fish vomiting, there are no indications she was a social outcast or that she was particularly anxious before this point.