What Causes Weakness And Fatigue?
I read your question carefully and I understand your concern.
Causes of weakness and fatigue may be many, so it is hard to pinpoint a definite cause from the present info. The first possibility to be considered is low blood pressure and heart conditions, so a measurement of your blood pressure and pulse rate is the first step. If it is too low and does not improve from fluid and food intake then you should present to the hospital for evaluation for possibility of heart issues like arrhythmia.
If that is not the case then it might simply be the initial stage of a viral infection which may initially manifest with fatigue. In that case there is not much you can do apart from drinking plenty of fluids and observing for appearance of other symptoms.
If symptoms persist then some blood tests would be necessary as well such as blood count, glucose, liver and kidney function, electrolyte panel, thyroid function etc for possibility of causes such as anemia, liver and kidney issues, low thyroid function etc.
I remain at your disposal for other questions.