I believe your first step should be to have a mammogram done. Either your PCP or your OB/GYN will give you a referral if you need one for insurance purposes.
Your results will be read by a radiologist who is trained to determine if your lump is benign or malignant. If the lump should turn out to be malignant, he/she will refer you to the proper specialist for further evaluation.
A lump in our breasts is very frightening and the best defense is early detection. Have the mammogram done as soon as you can.
I wish you the very best
RubyWitch
In the UK you would see your own doctor who would refer you to the appropriate specialist if he felt it was require.
Not all lumps are malignant.
It is always important to get any new lumps checked out and also any lumps that start to get increase in size.
A lump in the breast should be evaluated at a recognized cancer care facility such as Sloan-Kettering for evaluation as of yesterday. A primary care or Ob/gyn are not appropriate. That being said, a growth is not necessarily malignant.