Ferritin has two major functions;
it is the prime iron storage and carrying protein, a low ferritin is often associated with a lower amount of stored iron. Not sure what your Lab's reference range is for ferritin, a value of 50 May be considered a "normal" value. With a lower ferritin, taking iron may well increase your ferritin as more ferritin is needed to carry the extra iron.
Ferritin is also a reactant phase protein, especially to some types of infections. Your body could have been fighting an infection, won the battle, but consumed a lot of ferritin doing so - so your level was depressed.