Hi,
These things can happen from time to time, and usually aren't serious. The swelling and soreness could be due to either lack of lubrication during rough fingering (a.k.a. dryness),and possibly due to added friction from the G-string. Next time, make sure to tell your boyfriend to keep his nails clipped, clean and smooth; you might also consider using a water based lubricant before foreplay to help ease any dryness. For now, keep your vulva clean by gently washing w/ water only, and wear loose fitting cotton underwear or shorts. You can also apply an ice pack (wrapped in a washcloth), to your labia to help reduce the swelling and any pain. If you don't notice any improvement in the next few days, have it checked out by a doctor.
Good Luck