The definition of bipolar disorder has really expanded in the last ten years, or so. The definition used to include what we now think of as a "classic" mania, which is euphoria and grandiose thoughts. Now hypomania is included which can present as agitation, anger, irritability as well as the racing thoughts, sleep problems, etc. It is now thought of as a continuum of symptoms, not as two distinctive poles of mania and depression.
It is very common to be in denial. It is estimated that most individuals take 10 years before they go in to be evaluated. Oddly enough, the part of the brain that is affected is the very part of the brain that would normally let you know you were "ill." Because of this, many people who have BP never believe a single thing is wrong with them.
You are very wise to take your doctor's advice to seek further evaluation and treatment, if need be. It is a BIG step, and the most important step to getting on the road to recovery. BP is the most treatable of all the mental disorders. With proper medications and therapy you will be able to manage your BP rather than allowing it to manage you.
I want to reenforce what everyone else said. Be open and honest. Don't waste your time by being anything other than completely open and truthful. I would suggest you write some notes to yourself before you go in so you will have a chronology of your symptoms, episodes, whether there are changes Spring and Fall or seasonal, medications taken and whether they helped, etc. If nothing else, it will give you something to hang on to while talking to the pdoc.
Welcome to the board. Please keep posting with your comments and questions. Feel free to read through other posts if something interests you. I learn something every time I come on this board, and I hope you will, too.

Tsohl