Thanks for the query.
you might have developed a fungal infection
It is called as tinea cruris
It is common in folds of skin like butt crack.
You need treatment with oral as well as topical antifungals.
Try to keep the area dry,
You can also use powder to maintain dryness.
Take some anti histamine
This will give you relief from itch.
Please meet a dermatologist
for exact diagnosis.
Let me know if you have any other doubt.
you can ask a direct question to me on this forum, following the below link.
Wishing you a good health.