Well, it could be transference. Where something of significance comes up from the past, a trigger or reminder of something that happened and gets put into the here-and-now. Transference can happen to client or therapist. I'm not sure what significance fish tacos holds for you or her though? Either that is important to you or she really did forget the fish taco conversation because it wasn't that important to remember. If your therapist was forgetting vital information that is needed for your treatment, then yes that would be worrying.