As long as the kava is stored sealed in an airtight container and out of direct sunlight it can last a couple of years without much loss of quality. You've been drinking a while right? - so you aren't allergic per se. Maybe you got a shitty old batch that had been sitting around open to the air for a while and already had mould in it. I had something similar in 2009 when I was working in Vietnam - had a nice session with some north pentecost roots a friend sent me, and the next morning my face puffed up and became incredibly itchy. I ditched the rest of the kilo bag and have never had this reaction since.