The lump in your armpit can be several things and I recommend that you speak with your primary care doctor
. It's possible to have swelling in the soft tissue of the skin; this would be similar to the swelling around a pimple or a sore. If you had an infection in the area (which is what your sore sounds like) you could have some residual swelling from that will clear up; the sore could also progress into an abscess
which is a fluid collection beneath the skin. This would feel fluctuant, and might drain fluid. Lumps that feel like they're in the skin are likely related to the infection and are either residual inflammation or residual infection. If the lump is below the skin and feels like you can move it around beneath the skin, this is likely to be a lymph node. Lymph nodes can become palpable for many reasons, including in response to an infection. This kind of lymph node will often initially be tender, be freely mobile, and will gradually go away. You can get enlarged lymph nodes if cancer spreads to them, including from the breast, in which case it would likely stay large. If you are still feeling a lump, I suggest that you make an appointment to see your primary care doctor and have the lump checked out and to have a breast exam. In general, if breast cancer runs in your family you should discuss this with your doctor and determine when you should start having regular breast cancer screenings.