Small purplish bump on my step daughter's gum?

My step daughter is 5 & 1/2yrs old & in the very back, on the bottom right side behind her very last tooth she has a purplish brownish bump on her gum, its about the size of a tick. I was flossing her teeth & she has a mouthful of blood just from regular flossing like I do mine & my other kid's teeth. Just regular brushing causes a pretty significant amount of blood. She also has grayish, black stuff on her teeth, cavities? Idk. I only have her every other weekend but when she is here I brush & floss her teeth regularly. What is wrong with her? I'm making a doctor appointment & my mother in law is taking her. Her mom has her the rest of the time so I don't know how well she takes care of her dental hygeine. What is the bump in her mouth?
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It is most likely a "gum boil". It is an abscess arising from her baby molar. It sounds like she has other problems also, like cavities and gingivitis. Healthy gums should not bleed when brushing or flossing. She really needs to see a dentist. Good luck.
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