Probably because your bicycle is either too long or short for you from the handlebars to the seat, or your handlebars need adjusted up or down.
Check the handlebars. The handlebars can actually be adjusted up or down. Unfortunately, this is one of the more difficult adjustments to make since everyone has different back flexibility and upper body musculature. In general, your stem shoudl be long enough so that the crossbar of the handlebar obscures the front hub when you are seated and your hands are on the break hoods or a road bike or on the grips of a mountain or hybrid bike. Honestly, the easiest way to adjust this is to go to a bike store and have the bike store folks help you out. Sometimes you may need to get a longer stem length than your handlebars have -- a worthwhile investment to keep your shoulders from aching.
Women Suffer Sore Shoulders the Most
Many women under 5'6" tall -- myself included -- have difficulty finding bicycles with a short enough top bar -- that is the bar that goes from your handlebar to your seat -- to properly fit them. Bicycle manufacturers have finally in recent years heard enough women screaming about this problem and have now decided to make bikes that are especially fitted to women.
My personal recommendation is a bicycle company in New England called Terry Bicycling. Their web site is: http://www.terrybicycles.com/. It was started by a vertically challenged woman named Georgina Terry. She was sick of her shoulders hurting and decided that, with her engineers degree and her retirement check, she was going to start a company that makes bicycles engineered to fit women's smaller sizes. She was the first to do it and 10 years after she started her company, other companies followed suit.
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