Very weak & tired after donating blood 12hrs ago, sob, & chest pressure when upright. easily lightheaded. currently on period. should i go to the dr?

Heavy period?: You're 20? It sounds like you are volume depleted and anemic - both due to acute blood loss. I suggest you drink a lot of fluid tonight, take iron or eat red meat (or both) over the next week and see if you aren't feeling better? They test you before they take your blood, so you weren't anemic 12 hours ago. Is your period unusually heavy?