I went to my gynecologist (due for a pap smear and checkup) and also went to my GP (also due for a regular checkup and routine bloodwork) and after exams by two doctors, two pregnancy tests and a blood test, and a particularly gory period (ughhhhh) it's official: it's totally possible to develop a linea nigra for absolutely no reason. My gyn said that it's something that happens frequently when on certain birth control pills and that stacking them can cause a slight bump up in the hormones and therefore cause the darkening, especially if one spends a lot of time tanning in the tropical sun (hee hee). And my hormone levels came back completely normal too.
The more you know! Hope this is helpful to any other superstars who randomly develop a linea nigra and are not pregnant and also don't have symptoms of hormonal problems.