I am having rashes, hope it's not an allergy

Due to some rashes on my body, legs and hands, I am now on a strict home cooked diet which must be bland and void of reused old cooking oil like the ones fast food eateries are using.
My food must not be spicy nor load with sugar, salt etc. In essence, no dessert for me until my situation improves.
I am also not allowed to drink my favourite kopi O or any cold drinks but instead drink lots of water and Japanese green tea.