Don't worry, low ferritin is very common indeed, especially amongst pre menopause women and girls, but others too, anyone with undiagnosed celliac disease or other malabsorption issues.
And it can cause a wide variety of problems that you may find get fixed with iron supps, including difficulties sleeping and tiredness. One piece of research found women have to get their ferritin up to 60 before they report their feelings of fatigue easing (this was double blind etc).
Most cheap iron pills contain ferrous sulphate and I find that revolting and hard to absorb. There is a variety of other kinds, look on the bottle for "easily absorbed" and try a range. Make sure they have over 100 per cent of your iron RDA if you have a deficiency and ask for another test in six months time or so to see if you have fixed the deficiency.
Take orange juice or similar vitamin C with them, but research the other "co-factors" that help. I hadn't realised vitamin A was essential to use iron properly so I take a cod liver oil pill with mine. And it may be that vitamin D is also helpful, some studies suggest this, have you had your D levels tested? I should think everyone in Britain is low at the moment unless they have been taking high levels of supps all winter.
Don't worry! Common! Fixable!