I think if she says it's OK, then it's OK. I can't imagine that she would say it without meaning it. And I'd bet that she knows you well enough to know that you wouldn't abuse the privilege, or feels that you might benefit from some extra encouragement to reach out to her when needed.

I get that it's hard to believe that though, on a deep level. I sometimes wish my therapist would let me know that it's OK to call if needed.