When you're ACTUALLY sick on your first day?

So, my first day at Uni is tomorrow and I'm actually pretty excited about it!

However, early this morning I woke up feeling very unwell and coincidentally found myself being sick, and haven't really stopped being sick since this morning. I'm not sure if I ate something bad or have just come down with a bug, but I do know I feel pretty awful.

I'd love to wake up tomorrow and feel right as rain, but as the day is going on I'm starting to face the fact that there's a possibility that isn't going to happen and that tomorrow I may still be being sick, and may be contagious if it's a bug. I get pretty bad anxiety and have a bunch of horrid scenarios going through my head, I'm almost certain my lecturer isn't going to believe me, because why would they? Even I understand how suspicious this looks, and they don't know me so have no indication as to whether or not I'm lying. I'm also stressing that I'm going to miss something huge and set myself back before I've even started!

So I guess my main question is how much am I likely to miss and how much am I going to set myself back if I miss the first lecture or two? Any tips on how to tell my lecturer and how to make sure I don't fall behind?