Yahoo has made drastic changes to their email. I am not happy about it at all. What you are describing, sounds like you have a preview pane enabled.
I went into my yahoo mail to see how to disable the preview pane and there are barely any options left there at all. Yahoo used to have many options when using their email. I'm not sure if the yahoo plus (paid version) still has many options or not but the free version doesn't. I can't find a solution for you right now. I will keep looking.
If you are interested in the yahoo plus version, you can get it by following this link: Yahoo! Mail Plus
Thanks for the replies. I did go into options and unclick the preview pane and it worked. Interestingly though, it did not seem like that was an option - but regardless it's back to normal again. I do not have the "paid" version of yahoo, just basic. Thanks for help.