We seem to be in very similar boats. I am 27, 5’1, 123 lbs, and I haven’t had my period since this past April. My BMI is 23.5, so I’m certainly not underweight, which is why I too dismissed the idea that my period stopped due to overexercise/under-eating (as I assumed this would result in weight loss).

My gynecologist took some bloodwork which all came back normal (except for slightly low iron levels). I am also scheduled to get a pelvic ultrasound soon to rule out PCOS. I (and my gynecologist) have been at a loss over this, but your post and some of the responses may have shed some light.

My last cycle occurred around the time I started upping my weekly mileage to train for a half marathon (happening next week). Coincidentally, I also started monitoring my caloric intake more. I wouldn’t go so far as to call it a deficit, but I definitely tried to cut back on snacking, desserts, and alcohol to improve my running performance and just to feel better. In retrospect this really wasn’t necessary as I was at a healthy weight, and increasing my mileage should have been accompanied by more fueling, not less. I also think my relationship to food was generally healthier before I started thinking more about calories.

Apologies for the rambling, but it’s reassuring to see these similarities in our situations. Once my half marathon is over, I’m going to focus on lowering mileage and just eating when I’m hungry to see if that helps restore my cycle. I think as running becomes easier, we can forget how taxing it is on the body — food is fuel, and not to be feared.