I Got A Rash On My Body, Today Started Itching,

I got a rash on my body, today started itching, and as I scratched a little, it got more redness and it's spreading.
I'm taking Bennadril, it got better , and with some corn starch on the skin, too. however, it's still spreading more.
How do I know if this is an allergy or a reaction to a food or a detergent with which I washed my clothes?