45 yrs female. Ferritin level 5 (unknown cause). 600mg ferrous sulphate day. Resting heart rate 56-59. Not athletic (though not overweight). Too low?

Low iron. Low ferritin is an indicator that iron stores are low. It will increase back to normal as your body processes the iron that you are taking. Having a slightly low heart rate is fine as long as you are not dizzy or light headed. It is much better to have a slightly low heart rate than a high one.