I would have the doctor do a thorough check of your blood sugar. My husband and I have issues with our blood sugar, causing these conditions. My husband is also very lean. The size of your body doesn't always determine if you develop any kind of blood sugar issue. A 3 hr. blood glucose/insulin tolerance test is what you should have done. It will help determine if you have hypoglycemia or any other blood sugar problems.
There are many things that can cause these symptoms. Are their other symptoms that you may be ignoring or less noticeable? Just asking because sometimes the less noticeable symptoms are overlooked due to the dominant symptoms. I am curious if their is low grade temp or anything else. The two symptoms you mentioned could be neurological, autoimmune, infection, pretty much anything.