I've asked my doctor about this and she didn't know much about it so don't count your doctor knowing about this either. When you eat very low in carb and thus burning lots of fat, the hormones released from the fat metabolism will offset your cycle. The good news is that it is normal and not detrimental to your health. Many women go through this when they are lowcarbing.. in time cycle will normalize.. sometimes not.. but it's not harmful..

Serre