Hi,
I understand your concern and will be helping you with your health-related problem.
Generalized feeling of lethargy malaise is not a single disease and it doesn't have a single reason there can be many reasons.
Causes might be:
1) Some medication you are using like
Benzodiazepine, beta blockers, antihistamines, decongestants etc can cause lethargy but as you haven't mentioned any medication being taken, in your question so will rule that possibility out.
2) Chronic mental stress.
3) Disturbed
sleep cycle.
4)
Postmenopausal syndrome.
5) Thyroid issues.
6)
Depression.
7) Anemia.
8) Chronic infections.
9) Giving in to a sedentary lifestyle.
If there is any possible cause you think in the list, which you might be having then you can work on that, reversible causes can be dealt with by you yourself.
I will suggest you visit your doctor if this continues, your doctor might want to run some tests to know about the exact cause before starting the treatment. There is nothing much to worry about this seems to be correctable.
Hope I have answered your query. Let me know if I can assist you further.
Regards,
Dr. Ivan R. Rommstein,
General Surgeon