The period tracker app called Clue!
I came off the coil back in October after being almost a year out of chemotherapy for Hodgkin's lymphoma, was told there's a good chance I'm infertile, luckily my periods came back within a few months! You can track all your symptoms and it works all sorts out for you like your fertile window, cycle length and variations, when you're due to start PMSing (very handy for the husband
)and when to expect symptoms associated with your period such as headaches, diarrhoea, nausea, bloating etc. Also you can record your temperature if you want a more accurate idea of your fertile period.