If you drink any caffeinated drinks (not just coffee, but sodas too), look at your weekend intake versus your intake, if any, from Sunday night through the time these headaches hit on Monday. You say the headaches start at noon (If I read it right, it's a little hard to tell); do you not have any caffeine (any form! ) Monday before noon, whereas other days you do?
Changes in your caffeine intake, if you have it regularly, really can affect you. My best friend wondered why she always had awful Saturday and Sunday headaches by lunchtime. She figured out that she drinks several cups of coffee in the mornings Mon-Fri at work, but on Sat. and Sun. she sleeps in until at least 9 or later and then starts errands etc. -- once she started having coffee on weekends as soon as she got up, the headaches stopped. (Yes, mamas, she has no kids!) But I would not advocate increasing caffeine; really I would advocate reducing it every day, having a little each day if you must, but overall declining until you can do without it. Caffeine can promote benign cysts in the breast and increase fibrocystic breast problems. Not all women are prone to this, but if you ever have unexplained breast "twinges" that come and go, caffeine can be to blame, since the cysts it promotes come and go too.