Yes it is worth seeing a doctor... I am not sure how much you have lost due to not understanding the 'stone' piece. but if the weight loss is noticeable by others and the way you explain your eating habits and other things you may be suffering from a major depressive episode.. if that is the case the doctor might be able to address this with medication and recommend a counselor for you to see to address the issues...
Depression is debiliating in those ways that it makes us suffer. It's more than just a feeling of sadness since it involves the brain. Lack of energy, sleep problems, alteration of appetite - that's just some of the symptoms of a clinical depression. The weight you have reached now is maybe just one of the reasons you should seek answers and relief from your GP. Depression comes in many forms and shapes but for every form of depression, there is a treatment.
This time of year, just to give you an example, there is less daylight, which for some leads to a seasonal affectve disorder (SAD).
If you should decide to see a physician about this, I am sure that he/she will listen attentively to you and help you find a way to feel better. There is much more awareness about mental challenges today than it was only 10 years ago.
I hope you get the help you need,