Burning sensation may be a fungal attack or carcinogenic. Try to avoid hardcore sex which may disturb your breasts a lot. Consulting a physician is advisory.
Cause for burning in ones left breast may occur for some before a few days of ones periods this keeps changing every month. Check this and if you still have doubts consult a doctor and take test like blood and so on do not ignore it. The fright in you may worsen your pain
You have large breast they feel burning. If you feel burning behind your left breast and get weak, it may cause in cardiac. Or it may cause Breast Cancer.