have you change any thing in you diet, eating something new this last week?  are you under more stress this week?  i kmow bad headachs can cause stress.  i find that "tention headach" made by excedrin helps those kinds of headachs.  it works for me.  did you cut down on coffee(if drimk it) not enough coffee for me can give me one bad headach.  if you have swcah it might mean not enough salt.  i find when it starts to get cold i tend to forget to drink enough water and salt.

i hope this might help you.