I have a lump under my skin on the left side of my chest, under my breast. it is growing bigger and is very sensitive and hurts to touch, what is it?

Infection: This sounds like an infection which usually becomes reddened and comes to a head(pustule, boil). Apply warm soaks and have your physician exam it.

See physician: Should see physician for examination and detailed history. Could be anything. Sometimes painful lumps can be related to infection , but sometimes tumor with assumption that there was no trauma. May need ultrasound for characterization. If not specific may need biopsy.