Dorri, this has happened to me a lot over the years. When I'm tired, it's worse. It's like I know I'll have to really concentrate to even hopefully come up with the word so why bother! I just laugh about it now. I am happy when people finish my sentences because I don't have to! 
So many times the word is on the tip of my tongue and I can't pull it up. It will come to me later...maybe minutes, maybe days later. And, if I do think of it quickly, I feel like I won the lottery! 
I think the scariest thing that ever happened to me was that my kids told me about a conversation we had and I had absolutely no recollection of it...and still don't! I thought for sure I was coming down with Alzheimer's but the doctor said it was fibro fog. This happened 14 years ago so the doctor was correct by the way.
Try not to stress over this. That will make it worse. Play games where you have to recall things...like Bridge or word games. That helps me. I also suggest a great book called The Memory Book by Harry Lorayne and Jerry Lucas. Amazon sells it and it's cheap. I read it years ago and use some of the techniques in it to help with the memory issue. But do keep your sense of humor over this. Here is a cute site that will give you a few fibro fog giggles.
http://webspace.webring.com/people/fc/cfsdays/follies.htm
Sherrine