A sunny room does not guarantee a sunny position for every pot. A cactus behind a curtain, on a low shelf or beside the window frame may receive much less light than the room suggests.
Observe the actual spot at different times. Note direct sun, shade from buildings and whether nearby plants have grown across the window.
Move a shaded plant with care
If the identified cactus needs stronger light, increase exposure gradually. A sudden transfer from a dim room to an exposed outdoor position can damage tissue even when the destination is suitable in the longer term.
Keep watering decisions connected to the new conditions. A change in light or temperature can alter drying time; it does not justify watering before the medium needs it.
Existing stretched growth may remain visible after the position improves. Watch the form and firmness of new growth instead. If you cannot identify the plant, avoid applying a desert-cactus routine to it by default. Forest cacti and desert cacti do not become equivalent simply because both were sold under a general cactus label.
Related reading: Growing Cacti: Potting Mix, Light and Temperature.