prompted to sing in iCloud and iMessages every time i turn on Mac Book Pro

In the last few weeks, every time I turn on my Mac Book Pro, I get a pop up asking me to sign into iCloud and I messages. After doing so, I go to my Messages-Preferences and Accounts to ensure my iMessages is active and it is not so I have to re-enter my Apple id . Once entered, iMessages is activated but when I go to Account - Details, it asks me a window comes up and says, To view account information of (iMessage ), enter Apple ID; I do enter my ID then a pop up comes up with "account details" It lists my username and below two spaces for my password which has dots to represent characters. I know that password is not my current ID password as it is shorter. When I type in my current password, it tells me I must use a password I have not used in the past year. I don't understand? IT is not like I am changing a password because I have not clicked on the button below called " manage Apple ID; where i can change password etc".

Anyone can help me on this?

I would be happy to insert snap shots of the windows but don't know how to do so using my macbook, only my iPhone at this time.