It depends on the frequency and severity, however I'd look into deficiencies like vitamins, protein, dehydration etc. If none of that checks out, maybe talk to your doctor about possible POTS

Hi! It might be anemia or low blood pressure, both of which can cause those symptoms. If those are ruled out I'd look into dysautonomia (the family POTS is in) like was said above which I know from experience sucks lol For now I'd drink lots of water and sit up and stand slowly which might help the dizziness and blacked out vision, I'd def recommend talking to your doctor about it

This could be a couple things! I'd look into iron-deficiency anemia first, because that's the easiest to treat. Pick up an iron supplement from the store (ferrous sulfate is the classic supplement, but it can cause stomach upset. I'd look for ferrous gluconate or ferrous glycinate/bisglycinate) and follow the directions on the bottle. Even if you aren't anemic, adding an iron supplement to your diet can't hurt, especially if you have a period. People who have periods need at least 18mg a day. The next most likely culprits are POTS (postural orthostatic tachycardia syndrome) and orthostatic hypotension. If after 3 weeks or so, you haven't noticed a change with the iron supplements, try an at-home "tilt-table" test. Lay on the ground for at least five minutes, breathing slow and steady, until your heart settles to its resting rate. Then, stand. (Have something to lean on in case your vision goes or you get dizzy.) Measure the increase in your heart rate, if it jumps over 30 beats a minute if you're an adult, 40 if you're an adolescent, it may be POTS and you should get your doctor to schedule an official test. Orthostatic hypotension is tested through blood pressure, so it probably isn't something you can test for at home and you should ask your doctor. (They should test for it if/when you go in for a POTS test, because orthostatic hypotension has to be ruled out before they can diagnose POTS.) If it is orthostatic hypotension or POTS, the likely recommendation is going to be an increase in fluids (lots of water) and a high-sodium diet (lots of salt). Hope this helps a little!!