My baby has a blister on her lip. could this be from nursing too often?

Yes: Yes, blisters certainly can be a feeding blister or feeding callous. It could also represent something else. If it persists or the child has other worrisome symptoms (such as fever, lethargy, poor feeding, etc...) then the child should be examined by a physician.

Yes: This is commonly known as a sucking callous and should not be causing any pain. No treatment is required. If your baby seems uncomfortable, they should be examined to determine the appropriate treatment.