administrator username and password not working

Hi all

I have a problem. When I want to make any changes in system preferences i have to enter administrator username and password, when i do it window shakes and it does not accept my details. I know that username and password are definetely correct because they work when i log in after rebooting my mac. I changed my password, did not solve the problem. I tried to create new account but to do this i need to enter administrator details which means I'm stuck. The problem started after i reinstalled system a week ago using recovery mode. Im using yosemite. please help