Is it bad sign if they find a lump inside your breast from a mammogram?

Lump. It may not necessarily be bad, but it sounds like it will require further evaluation. There are benign lumps like cysts and Fibroadenomas, and there are malignant lumps. So at this point there is not enough information to venture a guess. This may require additional mammo views, ultrasound, or possibly biopsy to get an answer.
Possibly. Depends on so many things: does it look suspicious (certain types of calcifications)? Was it there before? Can you feel it? Does it hurt? Was it biopsied? Certainly needs more information and evaluation!