More tired than usual

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So here lately this past week I have noticed I’m more tired than usual. I am not pregnant I definitely know that. I just had my period Dec. 9th and I have 10 days before my next period...well according to the app I have 10days. Not sure if I believe the app. Anyway, I’m a person who literally has so much energy and I usually stay up until 3am or 4am I will sleep 2-3hrs and get up when my son wakes up and be fine. But I have been really tired and actually going to bed early. If it’s not pregnancy or my period then what could It be?