I have itching and skin discoloration in myperineum and anus. I've been to doctors and its not an std. Can it be related to steven jhonson synd? Help?

Can be due. To excessive caffeine especially if the itching wakes you from your sleep at night. Start but eliminating all caffeine sources for 10-14 days and see if it is better.
See dermatologist. It is highly unlikely it is steven johnson. It can be contact allergy, a manifestation of diabetes , chronic dermatitis that has lead to hyper pigmentation. Blood teat are indicated for underlying causes and if possible oral steroids cow, d decrease itching. The increased color once itching is controlled will require topical medications and treatment over months to lighten.