Have you established which database's log file is growing?  Has anything changed in the last two days?  Maybe you've imported (or deleted or updated) a lot of data in one go.  Maybe you've started doing index maintenance.  Or perhaps you have an overnight batch process?  If you look at the date of the transaction log file on disk, that will tell you when it last grew - was anything happening at that time?

What do you get if you run this?
SELECT name, log_reuse_wait_desc
FROM sys.databases

John