it's great that she's been able to tell you his, and, at 11, I don't think it's that unusual to be having thoughts about sexuality, and if she finds a range of people attractive, she may be most comfortable describing herself as this. I think it's dangerous to dismiss her thoughts and beliefs, simply because of her age.
Thinking about it, maybe it's preferable for young people to think of themselves as pansexual, until they get a stronger sense of their preferences. Keeping all options open as it were. At 11, sexuality will be explored in a hypothetical way, through fantasies, crushes, and chats with friends.
She may remain pansexual - great, or she may decide she is straight/gay/bi etc. Many of us will also change throughout our lives, so it's important that children grow up knowing that sexuality is not set in stone.
As it's Pride month, it's a great time to talk about things, and reassure her that whatever someone's sexuality, they have a right to be respected, valued and loved.