Behavioral Change

Question
This is a really odd question but almost overnight, my 6 year old shepard mix has become terrified of our fireplace.  This would be the first time we've used it in our new house but we've lived in 2 other houses that had them and she would lay in front of them!  She will put her ears all the way back and start to shake until we let her leave the room. Lately, if she isn't outside in the yard, then she is upstairs in the bedroom.  I've heard that animals want to be alone and hide if they are not feeling well and I just want to make sure she isn't ill.  When we are outside she will run and play frisbee just like normal.  But if she is downstairs and we turn the fireplace on she gets very skittish and goes upstairs.  She seems just to be acting differently all together.  I am also 6 months pregnant and we are starting to get ready for the baby so perhaps it's in response to that.  I am just very confused.  She was always mellow but now it almost seems like she is depressed.  Any info you could offer would be greatly appreciated.
Thank you.

Answer
At 6 years her personality should be mature and stable.  Your pregnancy will affect her, but I don't see it causing the fireplace problem.  I guess the fireplace could make a noise or leak combustion products you aren't aware of.  A physical is always good for unexplained behavior problems.