5yo step daughter is in pain, and I'm not sure what caused it or how to fix it

I'm hoping somebody will have some advice! So we got the kids for our half of the week last night, and as always there's something wrong. (Last week it was lips so chapped they bled) well this morning SD5 complained of being really sore in her "private parts" I asked her what it felt like, and she said it felt like it was bleeding. I asked if she wanted me to look at it, and she said yes. Her entire vaginal area is bright red. I asked if anything happened to it (ruling about abuse since we don't know much about her mom's new bf. Really happy she said no.) and she said no but it's been hurting for a few days. I'm assuming it's chaffing from too tight pants/underwear. But maybe she's not getting clean/dry enough down there? Do I need to take her to a Dr? Do I put butt paste on it? I'm out of my element guys. Also I really wish her mother would take care of these things sometimes. via /r/Parenting http://ift.tt/2jh54j8