Doing a Full Backup after Vacation

I was gone Thursday - Monday. When I turned on my computer today, Tuesday, it went to do a full back up immediately, and said it would take about 5 hours. Normally a full backup takes much less time. Also it should have done b2_s4, not a new full backup.
Does it have something to do with having the computer off during the scheduled backup time (Thursday)? Or does it have something to do with antivirus program interaction? Thank you.
I just cancelled it and hopefully all will be well on Thursday.