Yellow diarrhoea, constant stomach and lower back pain, fever, fatigue. This is two weeks post gastro. What could it be?

Go back to doctor. Am assuming you underwent a gastroscopic procedure. In which case this has either not solved the primary problem or has somehow created a new one and the doctor who performed the procedure needs to reassess you.

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23f constant headache x2 weeks with no relief, nausea, fatigue, occasional dizziness, low back pain with some very mild low abdominal cramping/pain?

More information. You need to provide more information for any reasonable answer to your question. At any rate a complex problem like you describe can not be handled on line. I advise you to see a neurologist after an initial internal medicean examination.