Yay! Hello Sweetmama! I am nursing an almost 3yr old too!
I had a MC when she was 16mths and that is what led me to charting too! Its been 18mths ttc since then and I have a short luteal phase which was probably the reason for the MC (if your LP, time between ovulation and AF, is below 10 it can prevent implantation or cause an implanted egg to not hold long enough.
Opinions vary hugely as to whether a short LP is caused from nursing (nursing increases prolactin which decreases progesterone which in turn gives you a short LP). I believe it can and does but usually fixes itself by now so I am trying different things to 'fix' my issue without weaning (B6, Agnus Castus).
As you are nursing two it could be keeping your prolactin levels higher than when you got pregs whilst just nursing one?
Its good that your are starting charting as it can tell you if you have a short LP.