Getting headaches while eating fish high in mercury (tuna is one of the worst) is a classic symptom of heavy metal toxicity. Hair falling out, leaky gut and yeast are also commonly described symptoms of metals.
Do you have silver fillings (50-60% mercury)? They are one of the leading causes of heavy metal overload, but not the only one. Lead is another common metal which causes people problems.
Most doctors won't look for heavy metals as a possible health problem, so I suggest you find out more about it. It may be the real cause of your trouble.