I have constant nausea + headache, better when eating and after sleeping, dizziness, nausea in upper abdomen+constipation. Cause? Had diarrhea 2 weeks ago

Many possibilities. You could have a viral illness of the GI tract(gastroenteritis) or liver (i.e. hepatitis) or any of a host of other physical reasons for your symptoms. You do need to seek medical attention as soon as you can to determine the cause of these symptoms.