Thank you for your query at DoctorSpring.com
I understand your concern.
What is you age? Are you on your period now? Sometimes just before or during periods, some women can feel a small movable lump in their breast, and that is called fibroadenosis, which is a small swelling that arises due to hormonal imbalance. It automatically shrinks in size, after the period is over, only to reppear in the next cycle.
Normally the major breast swellings, like fibroadenoma, or cancer swellings will be present throughout and won't disappear as in your case. So you can be rest assured it is nothing serious, may be just fibroadenosis, which is again very common in most women, only fact is it is incidental and not picked up by most women.
Before a mammogram, i will advise you to visit a general surgeon, who can palpate all the quadrants of your breast,and that will also guide him better, about what needs to be done. He will be able to pick up any benign tumors such as fibroadenoma or plain fibroadenosis, and treatment can be initiated.
Let me know if i can help you further,