It seems you are doing everything you are supposed to. You are not pregnant. It is possible your cycles are changing (hormone imbalance?), or you could have had a cyst that passed. Keep your appointment with your doctor to get a true diagnosis of what could be happening. You also may want to keep documenting your symptoms and menstrual cycle information so you can clearly explain it to your doctor as needed.
Your menstrual cycle also could be affected by changes in sleep patterns, diet and exercise. Anything there?
Good luck, and let us know what you find out.