A diagnosis cannot be made in a few visits, especially personality disorders. Any therapist who makes a diagnosis so readily should be avoided. Misdiagnosis follows you, and can make accessing treatment in the future difficult. I have been there. A diagnosis needs to be made with care. Your story and feelings have to be explored, and over a longer period of time.

Often the initial opinion is wrong, because a client can appear nervous, going through a difficult time, is having a bad day, etc... All this can obscure who they are and what is really occurring with them. Some therapists get stuck on the presenting complaint, and refuse to consider other possible diagnoses. It is important to know, the therapist's office is one context. Your world outside of that office has to be considered. A true and accurate diagnosis accounts for that. Anyway, for therapy to be effective, the diagnosis must be correct.

The prodrome comment indicates to me that the diagnosis is not concrete. You need to be followed if mania or schizophrenia is developing. Note: Substance abuse can cause similar symptoms.