Unless you do some sort of repetitive motion with your lefts arm, you may find that you simply slept on it wrong, and it will clear up in a day or two.
could be that you strained your rotators cuff without even knowing it.
Try to figure out what you have been doing lately to cause this. My arm began to hurt and then I realized that it was hurting because of where my mouse for my computer is now located!!! To relieve the pain, try Icy Hot or some other muscle rub. You can also take some Ibuprofen to help with the pain.
But you need to find out what is causing this pain! Either you did or you are doing something different physically, than you normally do OR it is stress!!! Are you finding yourself in a more stressful situation, maybe on your job, at home, etc???
Put lots of Sportscreme on it, especially right before you go to sleep. If that doesn't do it, see a doctor.
Eat some vegetables