Why am I getting dizzy?

I am over 15 months out and lately I have been noticing if I am sitting down or laying down too long I get dizzy about 5 seconds after I stand up. I drink 3+ liters of water a day so it can't be a dehydration issue. I am also taking in 1000 calories, few carbs and 100+ grams of protein a day. I am also faithful with my vitamins. Any suggestions would be much appreciated.

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