It kills me that I work myself to the point of total exhaustion and eat totally clean, low-carb and haven't lost ONE SINGLE POUND! I'm royally peeved about it.
Make sure your thyroid is COMPLETELY normal. Even being a little bit off can result in this kind of problem. Most doctors only check TSH, but some people can have thyroid problems even with a "normal" TSH. Check out the stopthethyroidmadness website for more info.
Other things that can cause a super-low metabolism: - iron deficiency (or anemia of any kind) - Cushing's syndrome (very rare - abnormal excess cortisol) - estrogen dominance (too much estrogen relative to too little progesterone) - toxicity (pesticides, mercury, pthalates, etc) - prescription drugs (anti-histamines, beta-blockers) - pre-diabetes/insulin resistance (although a low-carb diet is often the solution to this) - food allergies/sensitivities (if you're already low carb, try going completely gluten-free - this helps a lot of people)