Irregular periods/only bled for one day

Since having my baby a year ago I've had very irregular periods. It makes ttc impossible because my fertile days are all over the place. This month I only bled for one day. Pretty sure that's not normal? I saw my gyno right before we started TTC in October and she said everything was fine and ready to go. What causes this? I'm not overly stressed or anything. Feeling so confused and defeated.