Blood on toilet paper after i poop?

Im a 17 year old girl and I just went to the bathroom and pooped and when i pooped it didnt hurt but I am constipated when I wipped there was blood on the toilet paper and my poop was really dark. This never happens and now im extremely light headed. Sorry if this doesnt make sense I just cant really think straight... show more Im a 17 year old girl and I just went to the bathroom and pooped and when i pooped it didnt hurt but I am constipated when I wipped there was blood on the toilet paper and my poop was really dark. This never happens and now im extremely light headed. Sorry if this doesnt make sense I just cant really think straight right now. Someone please help me idk what to do
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