Skype default log in name changes itself

Hi, On my Skype there are 4 log in names, two are mines and 2 are ones friends have used in the past when using skype on my computer. I only use one of the log in names and usually it is always the default one. Recently however, I opened skype and kept putting in my password but it kept saying it was wrong, after 5 minutes I realized that the username that automatically appeared was my...