Lump in breast has grown a lot in 1 week. what could this be?

Get it checked: A fast growing lump like that is often a cyst or other benign (non-cancerous) lesion. Any lump should be checked though and i definitely urge you to go in for a mammogram and sonogram. A fast growing cancer is not unheard of, so please be certain by going in for care.

May need biopsy: A lump in the breast that grows quickly is usually not cancer, but should not go unevaluated. An ultrasound +/- a biopsy would allow you to get clarification quickly. In many breast practices, this is done in the office under local anesthetic.