Hi there...I can think of a few possible reasons for a lump near your cat's lip.
The first possibility is that it is something called an eosinophilic granuloma complex. It is a big long name for a condition that is usually not too serious. We don't know what causes them but some cats can get either a lump or an ulcerated area on the lip. They usually go away after one or two steroid injections. Some cats who get them repeatedly may need repeated steroid injections. For most cats this is safe, but there are a small number that can have side effects from the repeated injections such as developing diabetes. That being said, most cats that I see with this problem need a single injection and do fine.
The next possibility is that it is an area of infection or a tiny abscess if she has had a wound recently.
It is very unlikely that this is a cancerous growth. While they do happen in this area they are rare.
It is probably a good iea to have your vet take a look at her. They should be able to tell by looking if it is eosinophilic granuloma complex or not and suggest the right treatment for you.
I hope this information helps! If you have more questions, please hit reply. Or, if my answer has been sufficient please hit accept. Thanks!