What Causes Red Rashes In 4 Year Old?

MY 4 YEAR OLD HAS EZCEMA AND SEEMS TO HAVE RED RASHES IN BETWEEN HER TIGHHS, GROIN AREA AND BUTTOCKS, WHAT COULD THIS BE, IVE USED HYDRCORTISONE 2.5% CREAM WHCIOH SEEMS TO WORK BUT I WANT TO KNOW THET REASON WHY IT KEEPS APPEARING. SHE IS 3 AND A HALF FEET TALL AND 63 POUNDS.