The same thing happens with me. It's because, well both because of nutrition stuff... like because your heart doesn't get enough it doesn't get as good circulation and because of that, when sitting up suddenly the blood flow doesn't get to the brain fast enough. Take it slow and if it get really fuzzy and stuff lay down so the blood can get to your brain. Otherwise, you'll pass out and it's not fun. (you've probably experienced...) but yeah
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