Do you think that the many calls she makes have something to do with her keeping the relationship with you secret? Are you thinking that she is in another relationship with someone to whom she makes these calls? If your answer is yes, have an honest heart to heart talk with her about how you feel. Ask for answers to the questions you may have, and then decide on what's best for you.
If on the other hand, those are not your thoughts and you are only concerned about the secrecy concerning her relationship with you, ask her the reason(s) and again, make your decision according to what's best for you.
Because I do not have all the facts, I will also suggest that if you are very emotionally attached, you may need some counsel from a professional who can talk with you much more.