Is there a particular reason why you are not breastfeeding your 3 week old baby? Is she your first child? Have you noticed any breast fullness, engorgement, pain, fever, etc? Have you noticed any breast mass?
From your description, the armpit "nodule" seems unlikely to be of clinical significance, but we need to run a few tests to confirm this impression.
A breast and armpit ultrasound may be useful if this lump does not disappear in a week or so. Mammogram at this stage may be difficult to interpret because you are likely to have enlarged milk glands at this point.
All the best, and God Bless you both!