well blood loss is one possiblility... but there are others...
cutting releases hormones and chemicals in the brain, which give an almost high, but this wears off quickly... and so you can expiriance a "down". think of it like the feeling yo get after a sugar high.
could also just feel emotionally drained, that is a lot like feeling tired.
injuries can also cause tiredness as the body starts to want to sleep in order to heal. but this one affects people differently with some hardly feeling anything whereas others can literally fall asleep after any injury
there are other possible reasons too... but can't think of them in detail right now (ironically a bit tired so brain not working fully lol)
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