What to do? Soak in a hot tub of water for a few days, with some ebsom salt added. Sound like she just have a sore bottom. lol.. If her anus is tore, she needs to go thru emergency at the hospital for treatment right away.
Well, first is there any signs of blood? if so and it is still bleeding then maybe the doctors would be a first stop. If not then it will probably go away by itself, she just has to get over it. If your penis just happened (!) to penetrate a bit, maybe it is her muscles feeling a bit stretched but this will go away soon enough.