What Causes Lump Under Breast?

I have a very small lump the size of a pea under my left breast. Should I be concerned? It has been there for about 4-5 months. I just recently got health insurance so that is why I have not seen a doctor. There is no discharge, redness or swelling of my breast. I do feel some pain in that breast. I think the lump is directly in contact with my bra. This may cause some discomfort.