Could be anything, and none of us are PT's but here are a few things to consider.
*tight pec minor
*tigh overactive upper traps
*weak underactive mid/lower traps
I know this is really generic advice, but thats probably all you will get with the description youve given and over the internet.
More info would be helpful, like how and when did it start, what has your training been like, do you have postural problems, etc.