shoulder bothering new swimmer

I have been swimming almost a year and as I am getting "better" I have been upping my distance and workout frequency. I have slowly worked my way up to swimming three times a week and the distance is about 1500-2000yds a workout. I mix up intervals and straight swims. Mainly long swims in the summer and intervals in the winter. I think my approach has been rational thus far. However, my left shoulder has started to bother me. I have made adjustments to my mechanics and as long as I don't let it get out too far from my body I can deal with it. Right now my left arm is really only along for the ride so to speak and I can't really crank on it the way I would like to generate full power. I have talked with more experienced swimmers who have gone through shoulder issues. I am just working through it and hope it will subside. What is likely going on in my shoulder? Is it just inflamation? What advice can you give? Should I just work through it at reduced intensity? Any specific workouts in or out of the water to help? I am freestyle only.