tracking your cycle with birth control that stops your period?

growing
I recently went on birth control (Nexplanon) and have been spotting a little everyday for a couple weeks and then didn't get my period at all and I don't think I will. So on the cyclescope it says i'm on day 38 which there isn't anything wrong with that but if I never get a period is it just going to keep going up? How do you girls with the same situation track your cycle etc¿?