My belly skin is red. How can I make sure that my clothes are not causing this?

Allergy testing. It's possible to be allergic to certain materials called contact dermatitis. Also suspect could be detergents or fabric softener used on the clothing. A good first step is to get a patch test which is a panel of many common substances or chemicals known to cause allergic reactions. This is taped on the back and read after 48 and 72 hours.