Hello XXXXXXX
Thanks for posting your query.
Your symptoms could be due to varied causes like low blood pressure, hormonal imbalance ( thyroid disorder etc), nutritional deficiency , Hematologic disorder( malignancies,bleeding disorders or other blood dyscrasias) or a chronic infection like tuberculosis ( loss of appetite and weight loss.
Now to confirm the cause and severity of the problem it's very important to get yourself thoroughly examined by your doctor ( blood pressure , signs of nutritional deficiency or bleeding tendencies etc) & if needed a baseline investigation ( blood counts , ESR , thyroid profile etc) should be done.
This will help in confirming the diagnosis according to which further steps of treatment can be taken.
Yes , you do seem to be underweight but not malnourished . I assume your loss of weight and appetite are recent complaints ( less than a couple of months ) which could itself give rise to dizziness , headaches etc It would be wise not to neglect these symptoms any further and consult your doctor for a thorough evaluation and management .
Meanwhile you should try to avoid stress, caffeine and alcoholic beverages , and prolonged fasting to decrease your symptoms. Drink plenty of fluids to improve hydration which will help in decreasing dizziness too. Eat small frequent meals XXXXXXX in fruits (especially citrus fruits XXXXXXX in vitamin c )& vegetables . Your doctor might prescribe a daily multivitamin and calcium supplement if required.
Yoga or breathing exercises can be very helpful in the long run.

Hope I have answered your query.
Kindly accept my answer if you have no further queries.
Wishing you good health.
Regards

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