My daughther is having a painful lump under her arm. What is the cause of this?

How old is she?
Has she been ill recently?
Is she on any medication?
Under only one arm?

I am sorry I have to answer a question with questions, however, your question was so vague I cannot properly even begin to guess what may or may not be wrong with this limited amount of info.

The more info you can provide the better we can help you.