Unfortunately, our periods can go hand in hand with some pain. Odds are, the pain your experiencing is due to cramps. When I have cramps, the pain is usually right in my lower abdomen, but my niece used to say that her cramps felt like somebody punched her right in the vagina. Ouch! My advice is to be proactive. Eat healthfully, exercise, drink plenty of water, and (if your mom or dad says it’s okay) try taking some over-the-counter ibuprofen to help fight off cramps. The best defense is a good offense!
I’m sorry that your vagina has been so sore! When you have your period, your hormones are at their lowest, and that’s what triggers the period to come. But it also means that you don’t have hormones around to keep your vagina moist. Since this is your first period, chances are you don’t have heavy enough period flow to make up for that lost moisture and your vagina hurts. Next month if it happens again, try using a personal lubricant down there like KY jelly and see if that helps. I think it will get better with time and as your period gets a bit heavier, you won’t be sore down there.
This is not intended to be medical advice. Everybody is different so please make sure to consult your physician if you're having issues. Do not delay or refrain from seeking professional medical advice from your physician because of something you have read on this site.