Before anyone screams "it's COVID!!!!" - always possible, but not as likely as you'd think. I'm pretty much in the optimal effectiveness space of my last booster, and I am sick enough that I would have expected the RAT to show something.
(When my daughter was similarly ill, that fucker was red the moment it touched the line.)
But of course, I AM very vaccinated, I am an (on paper former for US insurance reasons) asthmatic, and viruses mutate, so it's no guarantee. I could have a low viral load and still have disproportionate respiratory responses because my body would really like to kill me. Or save me. Can never tell.
So I am home for the duration because I give a fuck about other people, COVID or not, and I am masking for folks that have to be around me here.