PS to previous post. I posted before I read the whole thread. I see some posters have already asked the same question, and there have been some responses.
I notice one person mentioned sensitivity to light, but I've seen numerous pictures of Mrs. Bush in all kinds of indoors venues and she never wore sunglasses indoors before that I can remember. She's often been photographed in places with very bright lights, under television cameras, and so forth, so why St. Peter's? It occurred to me that maybe it is allergies!!!! Maybe she is allergic to the incense?
All that incense can trigger a severe reaction in some people ....
Incense is an interesting theory. Vapors and strong aromas do bother my eyes when I'm wearing contacts.
But as some have mentioned, maybe she had been weeping or thought she might weep and didn't want to draw attention to it.