Fat lip

So my neighbors cat just had kittens a week or so ago and I've been doing SO much research on the pregnancy, giving birth, and kitten needs. I went over today just to see how they were doing and they look great! They are just starting to open their eyes, eating well, and all have lil fat bellies.
Now for the mom, I was looking her over and i noticed one side of her lip is red and swollen. I tried to open her mouth to see if it was cut or anything but i couldnt look on the underside. She did seem like it hurt a little and didnt want it to be touched.

I was wondering if maybe she could of just bit her self? Or if one of the kittens might have done it? Can cats get cold sores? Anything related would be helpful.

She could have been bitten by a baby, or her self. Or, it could be something called a rodent ulcer. Not from a rodent bite but it can be treated with antibiotics.

It could be a tooth prob too. Can someone take her in to the vet?

My kitty used to get a fat lip about once a year. Turned out it was a granuloma and she needed a steroid shot each time. They said it was most likely caused by an allergy of some kind.

Thank to all who answered. I finally went to see her today, noticed it was still swollen and she really didnt want me to touch it still so I told my neighbor what you guys said and she is planning on taking her to the vet this weekend.