Can a dr kind of tell whether a lump in the breast is worrisome or not? and if my dr thinks its benign why she send me for u/s

Yes and no: Doctors can suspect breast cancer but the proof comes from confirmation of lump being present and the final result/confirmation requires a biopsy of any lump discovered by means of hands or with an ultrsound/mammogram.

Sometimes: Some lumps are definitely concerning, others not so much. The ultrasound is for confirmation, since none of us have x-ray vision or know every answer.