I wonder/assume/think it is Glandular Fever.
I felt crappy for a couple of months when I was about 24.
I had a blood test and they said I was fine and should just man up.
Then 3 months later they apologised and said they mixed up the samples, and I did have GF after all !!
Glandular fever is a viral infection that mostly affects teenagers and young adults. It gets better without treatment, but it can last for weeks and make you feel very ill.
Check if it's glandular fever
Glandular fever is most common in people aged 15 to 24 years old.
Symptoms of glandular fever may include:
high temperature
swollen glands, which may be in the neck, head, armpits, elbows, or groin
sore throat
a rash, which may be harder to see on black or brown skin
headache
tiredness