Before you try any herbals, try to find out whether you have an allergy to something growing/used locally, a skin infection or are wearing the wrong clothing [fibers] for the climate.
According to some herbal sites, you can treat a skin rash internally and/or externally.
The internal herbals for skin conditions:
1- red clover [immune booster with some antibacterial properties]
2- sarsparilla [anti-inflammatory, anti-microbial action, and anti-diuretic]. This herb has some toxic effects and is not safe for everyone, esp. pregnant or breastfeeding women.
3- kelp - anti-infective
4- cayenne - anti-inflammatory, anti-infective
5- yellow dock - astringent, anti-inflammatory, anti-biotic
6- licorice root - anti-inflammatory
7- echinacea - immune stimulant [don't take if you're allergic to ragweed, marigold, etc. which are related]
8 - burdock root - diuretic, diaphretic [induces perspiration], anti-biotic and some anti-inflammatory properties
9- dandelion - immune-booster, antibiotic
Biggest problem is that the above list includes mostly European and North American herbs which may not be available in your area.
1- aloe vera
2- oatmeal bath/lotion [e.g. Aveeno]
3- onion poultice - mostly used as an anti-bacterial but has some anti-viral properties
4- capsicin - the active 'hot' ingredient from cayenne pepper, acts on the nerve endings to relieve burning sensation [key ingredient in shingles/herpes meds]
5- banana peel - the inside [white] portion of the banana peel supposedly contains ingredients that help soothe and feed the skin. [Not too sure about this because banana peel doesn't show up on very many sites.]