It can be pretty scary to have your stoma swell, for any reason, and not be able to get your appliance off over it for fear of doing damage. An appliance that hugs the stoma too tightly may cause it to swell. A fall or hard blow or a slipping appliance may also cause it to swell.
Rather than risk further damage to the swollen stoma by pulling the appliance off over it, try filling your pouch with ice water and letting it swirl around the stoma to decrease the swelling. Ease the appliance off carefully. Then replace it with an appliance having a larger opening until all the swelling is gone.