Why do I feel like I am going to faint? What's wrong with me?

am I dehydrated? I drank like diet soda, not too much water. My blood levels dropped about 1 gram, and my potassium is slightly lower.
So what is this? Is it dehydration, even though I'm not thin or is it only because of my period?
I get frequent periods and I felt pretty awful.
Update: I notice I feel better when I drink more water.
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