A period arriving as early as day 13, when you usually run 23 to 30 days, is unusual and worth checking rather than dismissing as a one-off. Stress, a hormonal shift, or a change linked to your retrieval cycle can all shorten a cycle this much even months later.
Next Steps
Track how heavy this bleed is and how many days it lasts, since that helps your doctor tell a true early period apart from breakthrough bleeding. Please book a visit with your fertility specialist to check your hormone levels given this is a new pattern for you.
Health Tips
See a doctor sooner if the bleeding is heavy, comes with pain, or if this early pattern repeats next cycle too.