I agree with what's been said here, and I'd like to add one more caveat:
The fact that she's started behaving differently towards you might be because the relationship between he and you is moving quickly (possibly more quickly than she'd thought it would) or it might have absolutely nothing to do with you.
And the thing is, a conversation between the three of you is essential to work this out.
If she says, "This is something in my life unrelated to you.", then you should believe her until proven that she is intentionally misleading you.
Relationships are always evolving and this may just be the start of a new cycle of evolution.