Skin Allergies after every two days.

Hi , This is Ubaid Shaikh , My Wife is Rubina Shaik . She is badly suffering from skin Allergy but we couldn't able to find yet why this is happening again & again , every two days later on third day suddenly whole body skin gets allergy ( like Mosquitoes bite every where) . This is from last one month & we really want to know why this is happening doctor (GP)asked us to find out which things doesn't suit you .but we couldn't able to find yet which thing is affecting or is there any other issue . On every third day she is having this skin Allergies suddenly any time morning afternoon evening . Once she take Avil or cetrizine she feel better .but how long she will do this hence we really want to know exact reason & treatment for the same . Kindly help me out.