dry, flaky and sensitive skin at the base of my spine

[Dr Stephen Kownacki]

From your description it is difficult to be sure what this problem is. If in the cleft between your buttock cheeks (natal cleft) with smooth red rash and scaly area on the buttocks than psoriasis could be the problem. If only on your back and not itchy then eczema is not likely since eczema always itches.
Moisturisers are helpful whatever the cause but you really need a proper diagnosis so return to your GP or ask the receptionist if one of the partners has a particular interest in skin disease then ask to see them. On- line many practices also list the special interests of the partners in a practice.
Failing that, request a referral to a dermatologist. but do not give up!