How old are you? Since how long have you noticed the lumps?
Underarm lumps are axillary lymph nodes which could be enlarged secondary to infection or even trauma. They are usually enlarged when a new deodorant is used, after shaving or waxing the underarm area.
Have you been on regular follow ups for your breast implants?
This could also be fat necrosis or scar tissue but nothing can be said with surety without getting the relevant investigations got done.
Have you got an ultrasound done as yet and have you got a clinical examination got done by a physician? This would help in coming to a confirmed diagnosis.
Let us know if you have any more doubts and keep us posted on how you are doing.
Let us know what your doctor advises.
Hope this helps.