A lump between your breasts but not in the breast
itself that doesn't go away could be a fibroma
or a lipoma
, an inclusion cyst
, or an abcess.
It is extremely unlikely to be a malignancy. If it is bothering you, an exam would be necessary to say for sure what it is but it should be fairly easy to remove and most likely could be done in the doctor's office if it is small. Hope this helps.