Read the change for what it usually means
Cats groom in a set sequence, washing the face with a paw and working steadily backwards, so an unkempt rear end on a cat who keeps her chest and face tidy is rarely laziness. Reaching the lower back, hips, and base of the tail demands a deep spinal twist, and that is exactly the movement that stiff joints, a sore back, or a rounder belly quietly takes away. Treat the pattern as information about her flexibility and comfort first, and as a brushing task second.
Take over only the section she cannot reach
Your job is to become her flexibility for that back section, not to give her a full groom she neither needs nor enjoys. Work where she already rests, ideally while she is lying stretched out or loafing in a favourite spot, so she never has to be picked up, turned, or held in the twisted position she is avoiding. Use a soft brush for the spine and hips in the direction of growth, then a fine comb in short passes through the thicker coat over the hindquarters where tangles start. A barely damp cloth wiped over the base of the tail and around the rear handles the greasy, sensitive area better than a brush, and lets you check the skin beneath as you go.
Stay ahead of mats, skin trouble, and litter problems
Ungroomed rear coats deteriorate in a predictable order: loose hair felts into small mats tight against the skin, skin underneath stays damp and irritated, and litter or debris sticks to long hair around the rear after tray visits. Comb the hindquarters before you can see mats, teasing apart any starting tangle with your fingers rather than pulling a brush through it, and never cut a mat out with scissors against skin you cannot see clearly, because cat skin tears frighteningly easily. Keep the hair around her rear trimmed short if she tolerates it, or ask your vet or a groomer experienced with older cats to do it, since hygiene in that area directly affects her comfort and dignity. Check as you work for redness, sores, lumps, or a smell that was not there before, and note anything you find with the date you noticed it.
Support the body behind the grooming problem
Gentle grooming help treats the symptom, so support the causes in parallel. If weight is part of the picture, weigh her monthly on the same scales and follow a gradual, vet-guided reduction rather than cutting food sharply, because rapid loss is risky for cats and a stiff cat who moves less needs a plan that still keeps her active within her limits. Make the rest of her world reach-friendly: a step up to her favourite chair, a low-sided litter tray she can enter without climbing, and food and water she can reach without jumping all reduce the daily cost of the same stiffness that stops her grooming.
When this needs a vet, promptly rather than eventually
Book a vet visit soon after you first notice the change if it is new, and do not wait for the coat to worsen, because loss of reach in an older or heavier cat deserves a proper pain and mobility assessment rather than home guesswork. Go sooner still if she cries when her back or hips are touched, stops using the tray, grooms one spot raw, loses weight quickly, or goes off her food. Bringing a clear record helps the vet judge speed and pattern, and keeping her grooming notes, weights, and visit summaries together in KinsLe Pet Passport means that story is complete and portable whenever she needs to be seen.