How come I have a large breast lump but mammogram and ultrasound show nothing?

Lump. Mammography and ultrasound are not 100% sensitive. However, if you have a large palpable lump it would be unlikely that both tests would be normal if it was a large cancer. It is probably normal tissue, but a clinically palpable mass shouldn't be ignored if it feels suspicious. MRI may help in this situation, or you may need biopsy. Talk to your doc about next steps.
Breast Lump. Negative results are good news. Unfortunately that does not guarantee that the lump is no a problem. Most lumps not seen on these tests are not harmful and either normal fibrous tissue or a normal fat deposit. However the only way to be absolute when a lump is not visualized is to have a biopsy done. I would suggest you discuss this with your doctor and have it rechecked in a month or two.