One of the things that I learn from Crossfit is that you have to listen to your body.
I got up early this morning to go to the gym. I felt ok but when we started I knew that I wasn't able to give it 100% and I was only able to a small fraction of what I wanted to.
In this case you need to listen to your body and no one else.
In a group lesson like Crossfit it can be quite difficult to leave the class but I learned early on in my Crossfit journey that you should leave your ego at the door. You are going to see people half your size lift twice as much as you do and that is ok. At the end of the day, you are there for yourself, no one else.
Listen to your body and it will look after you if you look after it.