Brief Answer:
Several possibilities.

Detailed Answer:
Hello and welcome to HealthcareMagic.

Feeling lightheaded and ill is not a very specific finding, meaning it could be a manifestation of many different conditions, so more info or tests might be necessary. Also it is not clear since when have the issues appeared and whether you have had any tests.

One possibility for light-headedness would be anemia, which commonly manifests with dizziness and tiredness.

Heart rhythm and function abnormalities would also lead to insufficient blood supply to the brain and other organs so would also justify those symptoms.

Another possibility would be metabolic alterations, like diabetes, altered kidney or liver function, electrolyte abnormalities. Hormonal causes like thyroid dysfunction or nutritional ones like vitamin B12 deficiencies must be considered as well.

If the symptoms have appeared only very recently it might be a simple transitory viral infection which are generally transitory and need only rest and hydration.

The list can go on for long, as I said those symptoms can be found in numerous conditions, only listed some of the most common ones.

So I would start evaluation with some routine cheap tests to screen for anemia, infections and metabolic issues like complete blood count, erythrocyte sedimentation rate, glucose, liver and kidney function, electrolyte panel, thyroid function, vitamin B12 levels, EKG.

I remain at your disposal for other questions.

---