gmail details column

All might be aware of the gmail details column which gives the details of your last 5 or 10 activity done (logged in sessions).

In one of the column i can see as :

Unknown United States (theplanet.com:74.54.240.26)

and i notification above says :

If the activity below doesn't look like yours, #-Link-Snipped-#.


Please tell me if there is any problem.I have already changed the password .But will it do any harm.???