I got a pain on my left breast since a month. it's really painful. i am worried if its a breast cancer.

Not worried, but...: ...If the pain persists beyond one cycle, please make an appointment to see your gyne or primary care dr. Depending on your age, most cases of breast pain are due to hormonal effects or chest wall pain. While it is true that most breast cancers are painless, some do--therefore, a careful breast exam +/- mammogram+/-ultrasound is warranted.

Common problem: Fibrocystic disease/changes is very common in women. This is most often seen during the menstrual cycle. Breast feel lumpy and painful which is caused by cyst formation (fluid filled areas) due to hormonal changes. Some women may have these changes without being on their menstrual cycle. Caffeine may cuase the cysts to be overactive resulting in pain.

See your doc..: Breast cancer is not usually described as painful and you are a bit young for breast cancer. This is more likely to be a benign condition, but stranger things have happened. If you are really worried about breast cancer there is only one solution. See your regular doctor for an exam and further studies if indicated.