HELP! Post appears with an other author’s name instead of mine?

When ever I post an article on my blog with the ‘admin’ account. It creates the post with some other author’s name who I have on my site by default. And it does not even show my name in the drop down menu at the bottom. What is this? Its happening since ver. 2.5.1. I thought updates might fix it, but its not getting fixed. Please need urgent help.