My anus itches really bad a lot, what could it be from. i'm 20 years old and really clean. any ideas?

Excessive cleaning?: Literally thousands of different species live in that area so dont be too clean. Excessive rubbing can irritate the skin. Clean gently but not excessively and give it some time.

Fungal Infection: It is common to have yeast/fungal infectio in the rectal area. Having hemorrhoids could be another possibility. Excessive sweating and wearing tight underwear can create a wet humid environment for fungus to grow. Keep area dry and aired out if possible. Avoid prolonged sitting if you can. Use over the counter antifungal cream like lotimin. Apply the cream at least twice a day.

Other causes: This can also be from hemorrhoids or pinworm infection. Get checked out by your doctor to determine the actual cause.