I have an extremely itchy bottom, which becomes worse when itched and doesn't go away when not. it spreads quite easily. what could it be?

Nonspecific: Physical exam is key. There are several conditions that can cause similar symptoms. Yeast infections, like candidiasis,is one of them, allergic rashes, such as hives, sometimes even a mild bacterial infection could cause these symptoms, and some viral infections could also cause this. So basically it could be any of the above. Seeing your primary care doctor would help get a diagnosis and tx.