Someone in my office has access to my email using my username and password...
I know I could change the password, but how can I find out when or who or from what computer my mail is being accessed. Is it possible to track their ip address or is there some other way?

The owner or IT manager of your company should be able to see who was on that web address at specific times.

Beyond that the internet provider would have to do the check for you ( don't hold your breath)

Just change the password before they do and lock you out of your own email