Does she have your password? If so, change it. Are you signing out of Yahoo when you're not using it? Make sure you do. Yahoo has a little box that's automatically checked off to KEEP you signed in. If you don't UNcheck that box (it's right under where you type your password), your account stays wide open for anyone using your computer to see. Next time you sign in to Yahoo, look for the box that says "Keep me signed in...", then UNcheck that box & always sign out when you're done using Yahoo. That should take care of it. :)
She obviously logged on to your account. I recommend you log her off then change your password. She WILL get mad though and demand to know your password, when that happens say this "I refuse to answer under my 5th amendment rights." And then she will get even more mad, but eventually give up.