I was masturbating a little roughly one night (I was doing clitoral and g-spot stimulation) and my labia became inflamed and swollen the next day. Should I be worried and what should I do?
A bit of swelling is really normal during sexual excitement. You get blood flowing into your vagina and vulva, so they can look darker and bigger on the outside and your vagina might feel fuller and wetter on the inside. It usually settles down afterwards, but might last longer if you don't have an orgasm. (The vulva is the area around and in front of the vagina, on the outside: the 2 sets of lips, and the clitoris.)
Stinging and burning are not so normal, so I wonder if you've done something to make yourself inflamed. Did you use some kind of lubricant that you might have had a reaction to? The vagina and vulva can be sensitive, so it's best if you need to use lubricant to use something that's safe for that purpose. Water-based lubricant is safe. If you don't want to buy something, a vegetable oil like olive oil should be fine. Allergic reactions are pretty common in the genital area, so do a spot test first: dab a bit somewhere and see if you get a reaction before using too much of it.
I suggest waiting a few days for this to settle down. In the meantime, try not to scratch, rub or use soap. Just wash with warm water and pat dry. Don't use any powder or deodorants (now or ever!) on your vulva because it doesn't need it.
If the soreness doesn't settle down within a week, I'd suggest seeing a doctor. But it sounds like your skin has gotten a bit irritated and needs some gentle care for a few days.