I accept the OP's claim that there was hostility, since that is something you can see directly. Racism, on the other hand, is an interpretation of the underlying thoughts behind the hostility, but you can't claim to know someone's thoughts. One can establish racism based on a statistical pattern in multiple events, but rarely based on a single event, unless e.g. racist words are used. One should never make such a serious claim without serious evidence.

In this case, without justifying any rudeness, it is clear why the FA was rude. Her records did not match the person in the seat (different gender, at least) so she assumed you were self-upgrading (sneaking into a higher-class seat), which happens more often that one might think. (Just last month I found an economy passenger calmly sitting in my business-class seat.) So you were NOT the same as the other passengers except for your race; therefore your experience cannot be attributed to your race.