This is a common phenomena. If you are having physical symptoms like dry mouth, muscle tightness or an upset stomach try some limited physical activity before coming ot lab. Also, a nice drink of water before you speak can work wonders in settling your nerves. A final technique that I like to use is called visioning. When visioning, you imagine yourself in a future situation.
Many athletes utilize visioning to improve their performance.
He simply imagines himself at the plate facing a particular pitcher. He has a good idea of the pitches that will be thrown to him. This proven technique can help you be a better speaker. Imagine yourself in the speaking situation. For this class, in front of the class in your lab.