Yes, and both before and after being on meds. However, there is a difference between such fluctuations during episodes vs. "mood lability". Doctors also must be sure a patient doesn't have a differential diagnosis, but I'm not implying this latter for you.

What I refer to above can be read about at Is it a bipolar mixed episode, ultra rapid cycling, or just “mood lability”? | My Journeys Beyond Bipolar Disorder Within this article, is a link to an interesting page on Dr. James Phelp's webpage.