Few cats can resist a closing bathroom door. From your side it is a bid for privacy; from hers, a member of her social group has just sealed himself inside a room full of water, warmth, and smells, and she intends to supervise. The behaviour is normal, affectionate in many cats, and driven by several rewards stacked in one small room.
A Closed Door in Her Own Territory
Cats patrol their territory on a loose rota, checking that each room smells as it should and nothing has changed. A shut door interrupts that circuit, and cats are famously persistent about barriers: the room they cannot enter becomes the room they most want to inspect. Add that you have gone in there, at roughly the same times each day, and following you becomes part of the routine itself. Scratching at the door, reaching a paw underneath, and slipping in ahead of you are all versions of the same request for access.
Water, Warmth, and Smell
Bathrooms reward a cat's senses like nowhere else in the house. Many cats prefer moving water to still bowl water, possibly because running sources were safer in the wild, and a dripping tap or a sink you run while brushing your teeth is an invitation to drink and bat at the stream. Taps and pipes make small sounds that catch a hunter's ear. The room also stays warm and humid after a shower or bath, tiles and bath mats hold heat, and there may be a sunny window or a warm spot above a heated floor. Finally, bathrooms concentrate scent, yours and hers, and scent is how a cat reads the world.
You, Seated and Predictable
You are unusually available in the bathroom. You sit still, at cat height, with idle hands, no screen competing as hard as usual, and a lap or a bath mat throne on offer. A cat that enjoys company learns quickly that this is reliable one-to-one time, and some will head-butt, chirp, or settle on your feet to claim it. Viewed that way, the escort is a compliment with an agenda.
Giving Her a Better Offer
If the following suits you both, there is nothing to fix. If you want the room to yourself, make the alternatives pay better. Offer a wide, shallow water dish away from her food and refresh it daily so the tap loses its edge, and put a perch or a warm bed just outside the door for the cat that mainly wants proximity. A short, vigorous play session before your shower burns off the following mood in a cat whose escort duty is really pent-up energy. Avoid scolding or spraying her; she is not being rude, and punishment only teaches her the bathroom is stressful.
One pattern does deserve attention: a cat that suddenly cannot let you out of sight, especially with hiding at other times, reduced appetite, crying, or litter tray changes, may be feeling unwell or anxious, and a veterinary check is the sensible first step. For the ordinary bathroom shadow, routine is the whole story, and KinsLe's Care Teams helps your household share the play sessions, water refreshes, and tray checks that keep her day full, with each duty marked done so the routine holds whichever of you is home.