As you may have heard, there has been a large scale Gmail / Google Docs Phishing attack yesterday. The targeted users will receive an email, probably from someone they know but not always, that contain a link to a supposed Google Doc file the sender is trying to share with the intended victim.
In actuality the link takes the recipient to a page which will prompt the victim to allow access to their Google mailbox.
While this was a widespread attack that ended up in the mainstream media, attacks like this happen all the time.
Always be suspicious of links you receive from someone, even if you know them. Confirm with the sender that they did indeed send you that link or document.
If you even have a slight suspicion that your Gmail account has been compromised, change your password. Better safe than sorry.