There is not such thing as fluid sexuality, at least not in the way many might think of it. Those who can be considered sexually fluid are those who haven’t really found out their true sexuality. Their orientation isn’t changing, they were just not really sure about what they really are.
If someone changed his sexual orientation at some point in their life, it was because they weren’t aware of their true orientation back than and therefore just changed the label after knowing it better.
A study has found out that most people know their true sexual orientation in their 20s after having more liberations and giving the chance to explore. Also long relationships at a young age can limit your sexual exploration.
Women are more likely to be bi or open to same sex encounters then men.
Also having sex with the same sex doesn’t make you gay or bi ( which many automatically assume at first). There is a difference between sexual orientation and behavior.
Jessie J was probably in an exploration phase and without really being aware of her true orientation stated that she is bi. Probably the reason she called it a phase. But in reality she was just experimenting.