Remember that it’s really common for us bisexuals to have doubts about whether we’re REALLY bi or not: Flipping from “Am I just straight?” one day to “Maybe I’m actually gay” the other, for example. You’re not alone in this. However, having these doubts is the most bi thing ever! The way I see it, wondering if you suddenly feel exclusively attracted to one gender, and then thinking you’re exclusively attracted to another, just proves that you like more than one gender!
(P.S.: This isn’t meant to invalidate the questioning of anyone who is exclusively attracted to one gender in any way, but as a reassurance to bi people who experience this!)