Testing using oAuth2.0 authentication

When doing the tests for integration, where the authentication method is setup as oAiuth2.0 using postman, we are facing a peculiar problem. The token generated by the tool at times is near 100+ chars. When this access token is picked up and used for doing tests, the integration works However, after the stipulated time, when the token becomes invalid, postman is generating a token which has approx. 2000+chars. Now when this token is used for doing integrations tests, the authentication fails.  Unable to understand the reason for this behavior and cannot find a solution to ensure that all the test cases go through correctly.