It's almost certainly a problem with your backups at some time. Maybe a while ago, but could still be current.
Your log file will grow continuously until you back up your database. This is by design.
If you use simple recovery mode, a backup will truncate the log file. This will allow the physical space the log file occupies to be re-used. It will not shrink the log file - it will continuously recycle the physical file space. You can shrink the log file from the SQL console. Don't shrink it to the minumum value, as it will almost instantly need to auto-grow, hitting the performance a little. This auto-grow will be 10% of the original value, which will soon not be enough, and will need another 10% auto-grow. Rinse and repeat. Far better to resize the log file to something like the same size as your database file.
If you're OK with SQL queries, you can run this query which wlll tell you the status of your backups.
SELECT sysdb.name, bkup.description, bkup.backup_finish_date,
when type='D' then '** FULL **'
when type='I' then 'DIFFERENTIAL'
when type='L' then 'LOG'
end as Backup_Type,
(STR(ABS(DATEDIFF(day, GetDate(),(backup_finish_date))))) as 'Days_Ago',
ceiling(bkup.backup_size /1048576) as 'Size Meg' ,
cast((bkup.backup_size /1073741824) as decimal (9,2)) as 'Gig',
,datediff(minute, bkup.backup_start_date, bkup.backup_finish_date) as 'Mins'
,cast(cast(datediff(minute, bkup.backup_start_date, bkup.backup_finish_date)
as decimal (8,3))/60 as decimal (8,1)) as 'Hours', first_lsn, last_lsn, checkpoint_lsn
FROM master.dbo.sysdatabases sysdb LEFT OUTER JOIN msdb.dbo.backupset bkup ON bkup.database_name = sysdb.name
where backup_finish_date > DATEADD(DAY, -60, (getdate())) -- Last 60 days
ORDER BY sysdb.name, bkup.backup_finish_date desc