How do I get sick in one night? (serious)

Ok so I have my tuition's science exam tomorrow afternoon and I'M NOT FUCKING PREPARED. I had my PTM today (for Prelim-2) and I did fuck up my exams (the checking was very strict I at least deserved 90% and I got fcking 80%).

I really cannot give the exam tomorrow as it will be REALLY REALLY FUCKING HARD (my tuition tr sets papers damn hard) and my parents don't seem to understand the term 'hard'. they think that I didn't prepare enough.

Faking my sickness won't work, my mother will see right through it.
If it is stomach-ache, they will still force me to go.
If it is headache, they will still force me to go.

They will not let me go only if it is a fever.
One thing I can do is that I can get a fever or severe cold and they will not send me and will instead bring the paper from the tuition (or sir will send a pdf) and I can do it.

So how do I get sick in one night so that I do not go to exam tomorrow?

<repost of my OC from r/IndiaTeens>

update: 2am in the morning, and I have 2 slices of onion in my armpits, my t shirt is fully wet and the fan is at 2. it's fucking cold but I brought this to myself ig.