if your getting thinner without trying, it could be a thyroid disorder. u can go to ur pcp and ask for a thyroid lab. Its a blood test. Ask your family if it runs in your family. There are two types of disorders-overactive and underactive. Yours might be an overactive!
it sounds like you have hyperthyroidism. That means your thyroid is overactive. Ask for blood work
You definitely should start with your primary care doctor. He or she should be able to run a CBC, and BMP as well as vitamin levels and thyroid function tests.
Hope you get to the bottom of this and start feeling better!
It sounds like what is known as cachexia, and requires a thorough physical work-up to determine the cause. It could be something serious, such as cancer, or something that can be easily treated, such as a thyroid disorder.
If you have a good primary care physician, he should be able to guide you through the process.
Otherwise, I would seek a teaching hospital.
In any event, do not delay in seeking an M.D. to get evaluated. You don't need to see a specialist at this point. After the initial exam you will be in a better position to choose a direction.