Brief Answer:
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Detailed Answer:
Hello and welcome,

Without being able to see it I can only speculate, but it may be a pilonidal cyst, which occur in this spot, folliculitis (infection of a hair follicle), or a skin abcess.

If the area around the bump is very red, hot, and tender, then she should be seen by a doctor as there could be a spreading soft tissue infection.

Otherwise, soaking in a hot bath and/or using a wash cloth soaked in very warm/hot water a few times a day can help a local infection resolve. The moist heat increases circulation (and therefore white blood cells) to the area and increasing the skin temperature makes it less favorable for bacteria to grow.

So without seeing it I can't say for sure, but it is most likely trapped sebum or fatty tissue or a boil type of infection.

I hope this information helps. Please let me know if I can provide further information or clarification.

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Brief Answer:
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Detailed Answer:
It depends on what is going on, but sometimes a hair follicle can get blocked and then sebum accumulates instead of coming out onto the skin where you don't notice it. In people with pilonidal cysts, it can be due to a structural defect, and then sitting, obesity, other factors can increase the risk of it.

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