Yeah, I agree with @HALLIEBETH87 and @~Christina in that not every mood change is due to bipolar.

I think it's important to be able to distinguish emotion from mood because they are very similar in a lot of ways, but still distinct. It is perfectly normal for anybody (i.e., neurotypicals and people with ANY mental illness) to have emotional changes throughout the day, but for people with bipolar specifically, emotions are influenced by environmental changes and/or existing mood only, while moods are due to chemical (and, rarely, environmental) changes.

It is possible you're experiencing genuine mood swings, though, so I'm not dismissing your concerns at all. In fact, I think you should set up an appt with your psychiatrist, GP, or nurse practitioner (or whoever you see) to talk about your moods to see if you're having mood swings or just emotional disturbances. However, I know firsthand that sometimes meds can cause mood swings of any duration, or even exacerbate existing ones. I know when I've been non-compliant, my moods genuinely changed every couple of hours where I'd have suicidal thoughts and come up with plans, then 2 hrs later have grandiose delusions about how amazing I was and how I was "destined" to do XYZ. So, it was NOT emotional. But that was because I'd take my meds like every few days or at completely different times fo the day (e.g., 8am on Monday, none on Tuesday, and maybe 4pm on Wednesday, then 4am on Thursday, etc. etc.).

It is extremely rare for people to have ultradian cycling on their own (i.e., without med or med compliance issues). But again, talk with your dr to come up with a solution so that you don't have to continue to experience this.