yep, that sounds almost exactly what i've been going through...even up to 2 x 15 inches which made me freak out for about 2 hrs one morning..
i too have eaten a lot of sushi for the last 5 yrs. i live in vancouver so it's cheap and plenty...looks like the wasabi didn't help in destroying them, i thought it would but recently i've learned that it's only there for the protein breakdown. :-(