Could just be stress and tiredness, even though you do not normaly get that long a headache. This is a first time you are mommy to a todler AND a new born, maybe you are more tired than you think? :hugs: If it keeps up for more than two weeks, I would go see a dr.
It could be heat also - don't know how hot it is where you are, but I usually get those 4 day headaches in mid- to late summer. I hate them and I hate getting up with a headache.