FIRST read this article from the reputable website Mashable to learn how 400,000+ Yahoo, Gmail, MSN, Hotmail, Comcast and AOL [and other] email accounts and passwords were stolen and published, making them vulnerable to identify theft.
THEN go to this link in that article [this is safe; i did it] to see if your email login and password were compromised by the July 11 hack into Yahoo.com. [you enter only your email address, not your password.]
This is just one of ever more frequent thefts and publication of large numbers of email accounts and passwords.
THEN even if your email account/password was not listed as part of the theft from Yahoo, it would be very prudent to change your passwords now and often in future.