Breast cancer and lump in breast

I am 28 and a mom of four. I had a complete hysterectomy four and a half years ago and have been on 1.25mg premarin eversince. My mom died of cancer in her reproductive organs and her breasts. There are also many other females who have had various cancers on my moms side. I have been extremely consistent on self breast exams and know them well. I have recently been having burning in my left breast and found a lump about a week ago. My doctor however doesnot feel it. It seems to be there in some positions but not others. I feel it when standing and sitting but not laying down. I do not have to use much pressure at all to feel it. Should I be concerned or should I trust that my doctor knows more than I? I know my breast but she has the medical knowledge. I am confused. Do I need another opinion?