After coming out as gay, did anyone else start to second guess their sexuality?

For a hot second, yes. Perhaps because I felt by saying it out loud, I had cemented it as absolute FACT, and there was no going back to what people previously perceived of me, that I was now destined to a life as 'the lesbian friend/daughter/family/etc.'.

Sexuality can be fluid and it can change over time. At one point of time you could feel like you're bisexual person but later in life you could feel like a homosexual person, or the other way round.

It's something that happens sometimes, particularly if people around you keep doubting your identity and trying to make you believe that's not who you are or who you should be. First, it's important to ask yourself why you are feeling this insecurity, and then work on the reasons. If other people are making you doubt, remember that you're the only one who truly know how you feel, they can't know you better than you know yourself! If you realize you're feeling things that you didn't notice before, though, it's ok for you to keep questioning until you're entirely sure how you feel. Feel free to take all the time you need, what truly matters is that you feel comfortable with the way you identify yourself, whatever it is!