If you have entered your credentials, it is possible that your Google account was compromised.
We would recommend you review information relevant to Phishing. Here are some useful sites:
https://en.wikipedia.org/wiki/Phishing
http://www.usatoday.com/story/cybertruth/2013/05/17/phishing-scams-steps-to-recover-privacy/2193105/
Our email traffic is filtered by the Google Message Security service, which provides you with protection against dangerous viruses and spam. However, some spam may occasionally get
through to your inbox or be diverted to your Quarantine Summary or Message Center, so please always use caution.