M.G.
I get a painful lump in my armpit fairly regularly. I've had all the tests. Mine is nothing to worry about. From what you have described - I personally would not be too concerned, however I would still ask about it - especially if it doesn't go away. Mine comes and goes, tends to be with my cycle.
It could be an inflamed lymph node - has she been run down lately, sick, or even cut her arm/hand on that side of the body? It doesn't take much to get a swollen lymph node. My son once had a goose egg lump on back of his neck just from being run down from his allergies when we visited my sister for a day (she had a cat). His went away on it's own in due time. My kids have all had those at one time or another - they pass on their own.
I have a relative who works on the pediatrics team - what she always does with any kind of lump or streak or mark - is to outline it with a marker (sharpie) to see if it grows, and take photos on her phone. That way if she does go in, or if it grows or shrinks, she can compare it and knows what the lump/mark is doing.
Hope it goes away and you have nothing to worry about.