I'll BET something that touches her hair is sour. Things to check:
- Hair tie
I do this myself every once in awhile with my hair tie. It'll be dry when I put it in (no smell) but as soon as it gets wet (either from wet hair, or sweat)... WHEW! Oy! It takes awhile for the smell to be noticeable... but I'll pull my hair down and, ugh. Sour hair. Gross.
My son, otoh, sweats a lot when he sleeps. So his pillowcase, or both pillowcase and pillow can go sour. Even though I wash his pillowcase once a week and his pillow once a month; in warm weather, I sometimes need to go twice a week on the case, and every week to two weeks on his pillow itself. His hair will initially smell fine (out of the shower in the morning), but as the scents wear off and he starts sweating the smell from his pillow starts leaking through.
And I think most of us have had the "whoops" towel. Again, dry smells just fine. Then we go to wipe our face right out of the shower and ugh. Okay, tossing that towel in the wash with a bit'o'bleach! Kids, otoh, frequently just scrub themselves with the stinky towel.
Cradle Cap... which in it's early stages before the waxy buildup has coalesced enough to form dark patches smells MUSTY.