Please take your cat to a veterinarian as soon as possible! Your cat could have a foreign object in its eye that is causing the eye to swell shut and have a puss discharge. The foreign object could be something as simple as an ingrown or inverted eye lash that is causing severe irritation and possibly infection, or it could have a stick, piece of grass, carpet fibre, larger object lodged in its eye. The fact that its eye is swollen shut and has a puss discharge indicates a serious condition that could result in your cat losing its eye. Another possible cause of these symptoms could be that your cat has been fighting with another cat and has become injured, or that your cat accidentally hit its head on something, causing damage to its eye. If you have a litter box in the house, cats often end up with tiny pieces of litter stuck in their eyes. In any case, your cat needs to see a veterinarian right away.