welcome to healthcare magic
It is a allergic reaction but it will be difficult to point out allergy
.Please try this :
---write down daily what food or drink you had.
---write smallest details like was it when clothes were tight or you were against pressure
----write down on which day or time the rash
appeared & try to think what you did just between that 1 hour.
--do this practice for 15 days then sit down & cancel common things what you eat or drink in these 15 days .After cancellation few things will remain & it will be easy to find what is exact thing causing allergy.
I know it may sounds weird/funny but please understand nobody other than you can find this allergic cause because you will be the best person to find out what goes wrong with your body.Once the cause is found treatment
will be simple.