app store login not working

WHen I go to the App store or to apps that I have already loaded onto my iMac, it will ask for my password, which i provide. THe response is either "unknown error has occured" or invalid user/pw. So after a few tries i will go to the "forgot password" and change the password successfully. When I go back to the apps store I get the same messages.

Another error msg is the app was loaded by another computer, which didn't occur either.

I did check for viruses and malware, but it came back clean.

Any suggestions??

Thanks,

Rochelle