More info needed AFTER surgery?

I was approved & they had a reference #, and even told me what % insurance would pay.   However I got a letter in the mail from Humana saying  "in order to determine coverage under your plan, additional information is required.  We are in the process of requesting medical records.  This additional information is being requested from xxxxxx    Information needs to be submitted to our office within 45 days upon receipt of the original request.  We expect to complete our review of the claim within 15 days after we receive the additional information.  Your clams(s) may be denied if information is not submitted within 45 days.   Based on the terms and provisions of your plan, the information received may affect the benefit determination of this and related claims." If I was already approved, why do they need additional info?  (I never had a letter like this with any other procedure.)  Humana never could get their story straight. They would tell me it was covered, then tell me it is not a "covered benefit".  (However, I thought it went to medical review, and that is why I got the approval.) I did not get the approval reference # or person that my doc's office spoke to @ Humana and I did not get a letter in the mail. (I'm sure I can get the reference # if I need it.)