It is good that they checked for any findings suggestive of infection. If there were uncertainty about the possibility of infection, then microscopic examination or a culture of the discharge may help, but it sounds like there was certainty that no infection is present.
At her age, it is more likely that a painful lump in the breast is a cyst, even though it feels different than her previous episode. The issue is how to prove it when it cannot be seen on an ultrasound. There are a couple of options at this point. An advanced imaging test, such as CT scan or MRI, will frequently be able to see lesions that canot be seen by an ultrasound. Another option would be to try to aspirate fluid from the lump.
If this is a cyst, it would be unusual to cause the armpit lymph node swelling or the swelling in the neck and shoulder if it is not secondarily infected.
The other option when there is a lump that cannot be explained by non-invasive studies would be to perform a biopsy.
If the current doctors are not willing to pursue further evaluation, it is appropriate to pursue a second opinion, but if it will take some time to get that appointment scheduled, you can try to ask her current doctors to at least consider scheduling an advanced imaging study of the breast.
If I can provide any additional information, please let me know.