It may not be always. I'd suggest seek for oncologist's advice as there are multiple test and examination required...some part of the lump is extracted and examined in the lab to find out if the cancerous cells are available in that in order to test.
A lump in breast may be a sign of cancer. A lump in breast can mean you have cancer. But this should be kept in mind that all lumps in breast do not signify cancer.
A lump in the breast may indeed be an indication of breast cancer; however, it should be understood that not all lumps which develop are necessarily related to a cancerous condition. In fact, most lumps develop due to benign causes, such as the development of cysts or hormonal imbalances which are a part of the normal biological process. Therefore, you should not panic unnecessarily upon discovering a lump, but it is still essential that you consult a doctor about any new or strange lump which you develop. Early detection of potential problems is essential for proper treatment, and a simple examination or imaging study is sufficient to provide some useful information about the situation, which can help to know what is going on and whether or not additional work is necessary.