My baby has little sores inside her mouth, and she cries when nursing; what should I do?

Evaluate. Mouth sores can be from a virus, bacteria, thrush or due to trauma or teething. No matter the cause, some pain relief is warranted. Simple tylenol (acetaminophen) or Advil are commonly used and frequently effective. Gentle oral hygiene such as OTC gly-oxide can be helpful. Sucking on a small piece of cloth-covered ice is a nice tip. If the sores worsen or pain relief fails, then a medical exam is required.
Viral infection. Get her check it may be a viral infection and the doctor may give you clues to how to feed her while sick.