Do I need to reauthorize my tokes in vb.NET


I’m sending out tweets in five minutute using the code below. It works the first few times, then I get an exception.
Do I need to reauthorize my tokens?

    access_string = InputBox("Acccess Code:", "prompt")
    isvalid = tw.ValidatePIN(access_string)

    If isvalid = True Then
        oauthtoken = tw.OAuth_Token
        oauthtokensecret = tw.OAuth_TokenSecret
        isauthorized = True
        isauthorized = False
    End If

     tw.AuthenticateWith(consumer, consumersecret, oauthtoken, oauthtokensecret)