One of my kitties started doing this, it seemed she was VERY fussy about a clean litter box. Ever if it was still relatively clean, she’d not want to use it for some reason – she would only use it when i put new sand in it, and then only a couple of time afterwards, and then she would wee on the bathroom carpets and poop on the bathroom floor next to the litter box. Maybe your kitty is really fussy about having a litter box cleaned more regularly (ie new sand)? Don’t get me wrong, my litter box wasn’t dirty or anything, but she was extra fussy for some reason – maybe some cats just get that way, hehe. Was you cat ever used to doing her business in the garden before, and now she’s started using a litter box recently?