Melora,
ITA with you!!! I understood exactly what you are saying.
I am one of those people who tires very easily and when I do, I am very quick to lose my temper. Fortunately for me, when I "lose" it, it really involves being extremely cranky and snapping at people. But if I don't get enough sleep and feeling that my life is one big treadmill, I fall apart fairly quickly.
I remember all to well the feelings I had after having both my children and returning to work. It was exhausting: Get up at 5:30 a.m., change/feed/dress baby, get work stuff together while baby is fussing, get baby to sitter, make 1 hour commute to office. Deal with office idiots only to come home to baby who 2 out 5 days a week was either sick, smelled like vomit, or was crying all the home. Get dinner together while feeding baby. Spend evening doing laundry and getting baby's bag ready for the next day. FORGET about exercise, FORGET about TV, FORGET about any time to decompress.
It's rough being a mom who works outside the home--or a mom that has more than just one child. You get ABSOLUTELY no downtime. This would make many people cranky, snappish, and extremely frustrated with their baby (and DH).
I do agree that it is wise to track the number of meltdowns and bring up how tired you feel with the doctor, but I'm not sure if this is automatically considered PPD. Sounds like she could just be exhausted. I have to say, unfortunately, in this day and age, a doctor will just throw some Prozac at you.