It appears that S may have an allergy. Worse, I suspect it is to our cat. :(
While I was still pregnant, Chris and I wondered if this would happen as we're both mildly to moderately allergic to cats. (Then why do we have one, you ask? Well, I "inherited" Lucy after my parents split up, and Chris "married into" her. Plus she's pretty much the Best Cat. But she'll definitely be the last cat we ever own!) We did some research into allergy inheritance and various sites suggested that if the parents have allergies, it is very likely that the child will as well (especially if both parents have them - chances could be as high as 75-80% in this scenario). However, the child will not necessarily be sensitive to the same allergens that the parents are.
The research also indicated that allergies probably wouldn't emerge until baby is at least a couple months old. Fast forward said couple months. Over the past few weeks, we've noticed S sneezing and rubbing her eyes a lot (though in the latter case, we thought it could have just been sleepiness). We wondered a little bit what this was about, but didn't think too much of it. Then, last night while changing her diaper, I noticed that she was scratching the top of her left leg. A closer looks showed redness and a little bumpy patch - very reminiscent of what I occasionally get on my hands and arms when Lucy's been licking or rubbing against me. Hives, I suspect. :( S had been lying in her Baby Neptune gym in a onesie shortly after Lucy had gotten into it to investigate, so the logic adds up here.
Soooo now what? Well, we'll start with a thorough cleaning and make a more diligent effort to keep the gate at the top of the stairs closed so Lucy can't get into her room. S has her 4 month doctor's appointment in a couple weeks, so we'll mention it then and see if allergy tests are in order to nail this down. And no more kitty bonding, unfortunately, as cute as it is! (At least until we get some answers.) If it turns out that S is, indeed, allergic to cats, we'll need to make a big decision. :( Here's hoping it doesn't come to that!