I see what you mean now. They are a bit transparent in places. Was she very young when she was pregnant? I'm wondering if she suffered a bit of malnutrition during the time frame those were growing in since it was an unplanned pregnancy and if just the stress of motherhood at a young age could be a cause.
I've seen quills like that before in a couple of my rescues. I've often suspected it was caused by some sort of stress during the quill's growth, or maybe as you mentioned, a bit of something missing in their diet at that time. I have a boy now, was part of a large number who were rescued from a hoarder, who has them. He has a 1 inch band of quills at the quill line to his fur on his rump, most are actually very orange as they must have soaked up some sort of dye and are stained inside. They have been falling out slowly over the last 7 months and he is growing in new white quills to replace them.