Back up time has doubled but hard drive the same size

I am using True Image 2013 and have been backing up for about a year. I have a 120g SSD with about 35 gigs on it ( my operating system). This for the year has taken about 9 minutes to back up on a 1 terabyte drive with USB 3.0 connection.
Last week it took almost double at 17 min. And today the same
I also back up another drive of about 180 -200 gigs and it took about 50 min. Now it takes about 1hr 45 minutes. Also the same today.
I am curious why the change. For the year every weekend it was always the same. at 9 min for C drive and 50 for the D drive. Now it is double the time it takes.
I do a single version of backup,
Any idea why?
Or is there a concern there.

Thanks