swelling on lip

my middle age cats bottom lip is swollen on only one side. does not seem to be in any pain. eating and drinking normally.sleeping and playing normally.

[Aleda M Cheng, D.V.M., C.V.A]

It may be a dermatological condition called Eosinophilic granuloma complex, or a rodent ulcer.  A vet would have to examine it and possibly biopsy it for a definitive diagnosis.  A dose of  an injectable steroid called depo-medrol often provides relief.