Yahoo mail hacked?

pineapple

I just received a few emails where the 'sender' was actually the heading of some of the emails in my inbox (!) and with an odd subject line re having a new boss. I deleted them without opening and changed my password.
Has the account been hacked? I've never put my address on the net though sometimes use it to order stuff.
And if someone has been able to capture the email headings in my inbox does it follow that they have had access to the contents, contact list etc?
Looking through the activity log I found two instances of 'Yahoo Messenger Authorization' today. I never use Messenger and am permanently signed out/invisible.
Done several virus and malware scans of the computer and found nothing.