You could always arrive covered from head to toe in a space suit or something,they probably won't ever invite you again ,just joking,I know this is a real problem,I am also sensitive to everything it seems,and forever have to miss special events,but usually because of lack of energy or feeling too anxious.
It's tricky when people don't understand or even try to
I suppose you have to be honest and firm.Explain the way it is & that's it.Explain to them the sacrifice you'll have to make to your health if you attend the event,will you be bed bound for the next 2 days...or how it will effect you. <img src="/ubbthreads/images/graemlins/flu.gif" alt="" /> <img src="/ubbthreads/images/graemlins/sadblownose.gif" alt="" /> <img src="/ubbthreads/images/graemlins/stretcher.gif" alt="" /> <img src="/ubbthreads/images/graemlins/ambulance.gif" alt="" />

You could always wait untill the day,&see how you are feeling.The way I see it is that you are not obligated to attend anything,you need to look after your health first,it's so important,and you should not be pressured into doing anything you don't feel comfortable about.