Is stupid to be afraid that my boss is back in town?

I got my job 4 months ago (approx.). My boss left on vacation 2 weeks later, and just got back tonight. So basically I don’t know him at all. I’m used to everyone else at work at this point, and I’m afraid to go back to work because I’m not used to my boss/the owner and I think it will feel like having a completely new job. The next time I work is Thursday. Should I act differently than I normally do, or just keep doing what I’m doing and try to get used to the new element?
I will be on my toes, what ever I do. I’m good at my job, it’s just that my boss is really sharp, where his son, who’s been in his place the last three and a half months is really soft spoken and relaxed. I am so nervous right now, I could probably start shaking if I thought about it hard enough. This seems like a huge overreaction to me, and I don’t why I’m acting like this.
What to do?