Lip swelling in cats can be due to following reasons.
- Rodent Ulcer: Also called eosinophilic ulcer. This ulcer develops due to irritation caused by allergy leading to eosinophil (A type of white blood cell) to invade the affected area.
- Allergy: There are 4 types of allergies in cats. These are, inhalation of some chemicals, food allergy, contact allergy and flea. In case of your cat, I think she is having a contact allergy with plastic and now her lips are swollen due to irritation. There is also possibility of flea bite.