I feel sick in my stomach and don't want to send text messages.

There's nothing wrong with it. It is human nature for you to do this, and you are embarrassed to tell the truth directly to the leader, which shows that you are a shy and kind person, so you choose to say that you have a fever. However, you didn't cheat him by doing so. You're really uncomfortable. I believe that if your leader is a reasonable person, he will understand you.