Having problems with a sore shoulder.?

Ok so I have had a sore shoulder for about a month. I'm only 24 and don't do much for strenuous activities so I'm uming I slept on it funny or lifted something wrong. I can't really lift my arm straight in front of me but have no problems with lifting to the side. I find it gets really bad and wakes me up in the middle of the night and I have to position my arm just right for it to stop bothering me. I find it gets a little better with some stretches in the morning but I can't get rid of it altogether.