Actually, your light is on because you probably have a leak somewhere, causing the excessively low level of fluid which probably triggers the brake light. I don't know if your car has a low fluid sensor to trigger the light. Generally, brake warning lights actuate because of a low pad or low pressure. The fact that you have such a low fluid level & the light might indicate a leak somewhere.

I agree to check the pads; however, you may want to invest somemore time in finding a leak. Could be an internal gasket: easy to fix if you have the tools and a Haynes manual.

Best of luck, bud.