Its very typical for people with Bipolar to have more than one diagnosis. Theres a word for it but, like you, I can't remember it. It seems like ever since the bipolar started, I can't remember anything anymore. I have BP and an Anxiety Disorder. Other people might also have a ... It just came to me!!!

COMORBIDITY!!! (I get way too excited when I remember things)

Theres quite a few other mental illnesses that can go along with bipolar and actually often do. He may be using Seroquel to treat bipolar and see if it helps your moods without telling you. (that may just be the paranoid side of me speaking... I can never tell if its logical or not)
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Goodluck. And look up comorbidity.