Why did my database size almost double over night?
Can't do much without more info; as a start run the following query on your database:

SELECT DB_NAME() as dbname, type_desc, name as logical_name, 
    TotalMB = CONVERT(decimal(12,1),size/128.0),
    UsedMB = CONVERT(decimal(12,1),FILEPROPERTY(name,'SpaceUsed')/128.0),
    FreeMB = CONVERT(decimal(12,1),(size - FILEPROPERTY(name,'SpaceUsed'))/128.0),
    MaxSizeMB = CASE WHEN max_size = -1 THEN NULL 
        ELSE CONVERT(DECIMAL(18, 1), max_size / 128.0) END,
    GrowthRate = CASE WHEN is_percent_growth = 1 THEN CONVERT(varchar(12),growth) + '%'
            WHEN growth = 0 THEN 'FIXED'
            ELSE CONVERT(varchar(12), growth/128) + 'MB' END,
    physical_name
 FROM sys.database_files WITH (NOLOCK)
 ORDER BY type, file_id;


This query looks into the detail of the database files, and tells you whether you are simply dealing with a lot of empty space, or actual data, and whether it is located in the data file or the log file.

You can also use the "growth rate" to determine if it simply grew in one big chunk, instead of smaller amounts.

Any further action depends on where the unexpected growth actually took place.