Yes, a person can be sexually fluid. This means that they may not identify with or have the same level of attraction to one gender as they did in the past, which can change over time. There is scientific evidence to back this up and it has become more widely accepted and discussed in recent years.
One key study was done by researchers at Northwestern University who surveyed 369 gay, lesbian, bisexual, and pansexual individuals between 16-20 years old between 2010-2017 (Savin-Williams & Ream, 2017). They were asked about their sexual attractions on a 6-point scale ranging 0 (exclusively attracted to people of the other gender) – 5 (exclusively attracted to people of their own gender). The results showed that only 14% reported exclusively homosexual/lesbian attractions while 48% reported mixed attractions and 38% reported exclusive heterosexual attractions. Furthermore, those who had “mixed” attractions were more likely than those with exclusively homosexual/lesbian orientations to report changing levels of attraction over time.