I know yahoo mail has been playing up for me occasionally too so perhaps it's a glitch, but they are also in the process of updating to new email (can't think what it's called) and they are inviting people to change over now rather than wait until it's done automatically. Don't know if this is anything to do with it or if you have already changed to the new one and it's "like that"...hope not as I wouldn't like mine to be like that. Hope you get it sorted and it's just a glitch atm.
I've been have huge problems with Yahoo - spoke to an engineer and apparently there is some updating with servers etc (don't really understand cos am a technophobe) but it may take some time!