I was very ill when I did my GCSEs so I had a doctors note. But the deputy head told me to sit the exams just for the experience and they wouldn't send off the papers. So I the exams, and what did they do? They sent off the papers! I was absolutely positive that I was going to fail and not get into sixth form since I hardly did any revision. But, come results day, I actually had pretty decent results! Not what I was expecting, obviously! But I came out with 2*s and 8As 2Bs!
So, seriously don't worry about it! If I managed to scrape through, somehow someway, you'll be absolutely fine!! GOOD LUCK!!