Hi,
You should keep the area clean, wear lose clothes and see if the pimples go away in a few days.
Pimples on the breast can occur, just like on skin anywhere else.
You should do a breast self exam and report any abnormal features that you might notice - a breast lump, skin changes or nipple discharge etc.
If the pimples do not resolve in a few days consult your physician for a clinical evaluation.
The underarm lump you notice could be due to an axillary lymph node which could be enlarged due to shaving, waxing, infection, or a deodorant , also. Wait for a few days and see if it subsides or else consult your physcian. You could try some warm compresses.
Let us know if you have any other doubts and keep us posted on how you are doing and what does your physician advise.
Goodluck.
Best guess would be that you are probably feeling a lymph node that is reacting to the pimples--lymph nodes react to infections for a living. If you can get the pimples to clear up, the lymph node should then shrink back down. If it doesn't, I would get it checked out. Antibiotics from your general doctor may speed the resolution of the pimples.