Just did a case study of this (Haematology/Clinical Chem).
Could be anything even malnutrition or liver infection.
Thing is you need tests, correct cause and treatment.
What you don’t need is a **** forum full of suggestions.
Relax (ok it’s easy to say when you’re not ill but you can’t do a thing and worrying gets you know where but adds stress.
Wait for tests, look up info on NHS direct if you must and what ferritin is for so when you go back you can ask your Gp more questions.
Good luck and hope you get sorted asap.
Go have some fun and laugh.