pomeranian panting

I have a 4 year old toy pomeranian.  She has always been very dramatic and can put on a good show, but I am beginning to really get worried.  Autumn began panting during the night when she was 2.  The house was air conditioned in the summer and was cool and not overly warm in the winter.  The panting has gotten to the point that my husband will no longer let her sleep in our room because it keeps us up at night.  Also, she snorts like she can't get her breath.  She is overweight and we are trying to help her with that.  We only give her Iams dog food and a pupcorn treat 2-3 a day.  It is very difficult to walk her as we live in the mountains.  We are deep in winter and her little legs and feet can only take so much.  She will just sit down and refuse to go on.  She will only play fetch for a few minutes, before she is exhausted.  I am wondering if it could be her thyroid or something with her trachea.  This spring we can get her out for daily walks again, but until then should I be as worried as I am.  She is my baby, of course and means the world to me.