There's not enough detail here to allow me to figure out what's happening here.
When you read an email, which name changes? Your email address?
What can't you find again? The same message?
And when you are reading these (changing?) messages, what sequence of buttons or commands are you using?
As a guess, if you're viewing messages in the middle column of the three-column layout, there's a small header at the top of that column with what's called a disclosure triangle. If you click on the disclosure triangle, you'll get a series of nine details that you can sort messages by, and two additional items that allow you to choose an ascending or descending sort. That's usually set to Sort by Date and Descending, for general viewing. If that sort changes, then messages will be sorted by what is selected, and in the order selected.
If you're new to OS X, then I'd suggest spending a little of your time with the Mac 101 to get the most out of your new computer. (That area is useful even if you've been at this for years, as Apple can put some useful new tips in there; things that I might not have found on my own have been discussed there.)
