Is this lump sore on touch?
What's the size and color of the lump?

when you press it, it hurts. It doesn't have any color.

Probably a swollen lymph gland but I would get it checked out anyway.

Your daughter's armpit is probably swollen, what's the size? Also, get it checked out.. you can never be TOO sure.

Evan small scratch on her arm can cause swollen, painful lymph nodes under armpit.
Also, children and young adult tend to have a more active lymphatic system, so their glands may feel enlarged.
However, your daughter should be checked by a physician.