I had sweat rash last week. under my belly (am over weight) i put cream on it (diaper rash cream/sudacream) it helped.. but the rash has turned brown is the normal? it was very hot weather so it definitely was from heat, and sweating.. but is it ok

Can be normal: It is hard to tell for sure without seeing it (you might need to consider a video consult or in person visit) BUT when the skin is inflamed with a rash there is frequently a period of increased pigmentation/dark color called “post-inflammatory hyperpigmentation “ which May gradually fade (1% hydrocortisone can possibly help). If it becomes itchy/painful may need Dr eval. Weight loss will help