Different bikes fit different body types better than others. If you are looking for a new bike, it's a good idea to try out a number of different ones and see what feels good. I use an upright bike because leaning forward seems to always cause fatigue in my back and tension in my neck and shoulder muslces. I don't know what style of bike you are currently riding, but it definitely could be a posture issue. If you aren't considering a new bike anytime soon, take the one you have to a shop and explain what you are feeling. An eperienced person might have some easy fixes for you by simply adjusting the seat and or handlbars. It could be as simple as leaning too far forward or something like that.