For the last week, I've tried to sign into my yahoo email (retrying several times to make sure wasn't typing it wrong), & I've had to put in a new password every time. The problem is, that new password doesn't work more than once.
It sounds like a spammer/hacker is accessing your email account. You need to go into your email and change your password AND your security questions. I would also change my alternate email address or make sure you have one set up so you don't lose control completely of your account.