Unable to find source of allergy

Patient: Hi doctor,i have itchy feeling for the past one month ,especially on the back of my body.cetirizine hcl tablets restict me from the itchy feel but when i stop taking them i am ending up with same thing. my doctor asked me to continue with the same medicine for next three month. also she couldnt find out the source of allergy or the reason for ichiness. please help me to solve this issue . it is getting bad day by day

Symptoms: Itchy feel

Doctor: