If you've ruled out a change of washing powder, new clothes etc, it could be to do with 'a filler' in one of your medications. Do the hives get worse so many x number of hours after taking one of your tablets? The only way to rule this out is to explain your hives to your chemist and ask for a different brand of one of your medications. If that doesn't help, try the next medication/different brand etc.
I was covered in hives that kept getting worse, and used this method to identify the culprit. With 2 of my meds I can't take a generic brand for this reason, only the brand name tablet.