Sometimes when my period is over I have severe rashes even its difficult to walk. In this above situation can we delay obligatory bath for one day or its a sinful act
This is not permissible.
You must take the ghusl and start praying as soon as you experience purity. If water would harm you, you can skip washing the affected area and wash everything else and then make tayammum for the skipped parts.
You can use medically safe creams or consult a gynecologist for your issue.