I Have Pain In My Left Shoulder When Reaching Across

i have pain in my left shoulder when reaching across my body. it has become progressively worse over the course of the past three to four weeks. Sleeping on it is also very painful as well, causing me to wake when I roll to this side. there is less pain when I lift my arm, mostly jus when reaching across body and sleeping on it. I have not had an injury or anything else recently that would have been a cause of this. I have had problems in the past with this shoulder but not in the past two years. I have tried using the exercises I used when I did physical therapy previously and they are not helping. what could be the culprit of this pain?