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Many a times mono infection lingers and causes painful glands to persist. Hence an antibody test for mono should be done to know if there is any active infection. The other possibility is super infection by bacteria. Hence a throat swab should be taken. A CT scan of the lump should be done. Apart from this post viral fatigue syndrome is probably what you have and this is a very common disorder. Also there are no magic pills for it. This can be treated patiently with exercise, good diet, work out plans and multivitamins. A diet rich in fruits, vegetables and proteins should be taken along with a multi vitamin supplement. Eat 5-7 small regular meals every day. Drink plenty of water. Do deep breathing exercises.
Test for anemia too.
Hope this helps. Please let me know if there is any thing else and do keep me posted. Take care!