I agree with the other girls that it is that you are not fitting on your bike correctly, but want to suggest something else.

A simple thing that often times gets overlooked is some women have uneven legs (i.e. one leg is longer/short than the other). Make sure to check this becuase your LBS will most likely do your fitting to match as if you have equal legs and not suggest anything else, such as shoe inserts, etc. A friend of mine had the same exact problem and was able to solve it by riding with shoe inserts in her left shoe.

Also, make sure you are breathing correctly and are fully hydrated as that can often times give you those one sided shoulder pains.

Roshelle from Milwaukee
http://wisconsinbetties.blogspot.com/