User Logins Changing?

Yesterday morning I went to log into the back end of my site, and I received an invalid username error. I thought this was odd, so after trying a handful of more times, I logged into my web host, and found that the username for both of my accounts had been changed. I changed them both to something new, and I was able to log into my site.

All of the accounts were showing the same random username and password in my database.

This morning, I also received the same problem.

Anybody have any ideas what could be causing the user login names and passwords to be changing at random?