Need a V.good excuse for not drinking

Okay guys I really need your help. It's my works Christmas party on Friday night, I'm meant to be drinking however I took a test today and got a vfl. If I am pregnant I need to have a VERY good excuse as to why I'm driving. I'm a big drinker normally and a terrible liar. I don't want to feel pressured into telling them anyway so I need a bloody good reason as to why I can't drink. Antibiotics wont cut it, I need something way better. Any help appreciated