Hello and welcome to EmpowHer,
Hydrogen Peroxide is a bleaching agent and a disinfectant. Generally you should use it to disinfect a cut or wound that may get infected.
You should not use it on just any skin rash because you can make it worse. You need to stop putting it on your rash right away. The rash needs to be diagnosed by a dermatologist in order to know the proper treatment for it.