The warning message is high possible caused by too many access token that is generated. Actually there is limit up to 8 for valid existed tokens which would apply for most cases. If you obtain the access token through get_token method every time, access token will be valid for 30 minutes, if there are more than 8 get_token requests in 30 minutes, the threshold of valid tokens will be reached, and you must wait till expiration for one of the original tokens.
Therefore the idea is do not always generate access token by get_token method every time!
Please always use the access token which has been generated to post requests, DO NOT generate new access token every time.