What can cause a eally bad pain in my upper back left shoulder blade?

Commonly missed: Rib. The prior answer is absolutely correct. The only addition i'll make is that one of the most common causes for pain in this location which is not serious, but is extremely painful is a dysfunctional rib. Md's almost never make this diagnosis, but osteopathic docs who commonly do hands-on diagnosis and treatment, in addition to doing all the other work-up, often find and treat this problem.
Acute or Chronic? There are numerous potential cuases for this type pain (or any pain really). Could be a simple strain, or cervical neck issue which is readiating to your shoulderblade, but could also be someting more sinester. Not to scare you, but get reliable medical advice. After imaging the md can evaluate the cause and propose a treatment plan.