I have a linea nigra from about two inches up my bellybutton to the start of my chest/bra. I am not pregnant and would not be pregnant. What is this?

Could be hormonal. A linea nigra (literally "black line") can run either upward or downward (or both) from the belly button. Do you take birth control pills? They raise your hormone levels, like pregnancy does, and can cause linea nigra. If not, you could be secreting abnormal levels of hormones from your adrenal gland (or elsewhere), and you should see your doctor for lab tests.