Public Speaking

I am a natural talker.  I talk for a living and literally I spend most of my spare time looking for someone else to talk to and listen to.  Yet, last week our Pastor asked my husband and I to speak at church this week to give our testimony about a sustenance fast we had done a few weeks. Instantly my stomach turned.  So why is it that when you have to speak in front of a large crowd we get nervous?  Any answers?  Let me know.  I will update you on how it goes.