Recently my gmail account was compromised and was used by some china based spammer group who sent some spam promotional mail to almost all the contacts on my account. I have written a long article after that how that happened and how you can avoid happening the same or similar with your gmail account here.
Just in case you are not sure, if someone else is using your account and might be logging in time to time or logged in at some other computer or device. In gmail you can sign out other users signed into your account at any time and also get to know who else has been logged into your account.
Follow the procedure to know more, how can you track and find some one using your account.
Log in to your gmail account, once you are logged in scroll down to the bottom of the page and click details link as shown in the image below.
Once you click the details link, a small browser window will open up, which will show you all the IP addresses with which you were logged into you gmail account previously, if you see some red label entry like one shown in the image below, log out all other sessions other then yours which means that all others sessions will be logged out expect yours and then immediately change your gmail account.