I've been getting a prickly, warm feeling in my upper left back, what could this be?

Sensations in back. There are many possibilities. It sounds as if your nerves in that area are being irritated somewhere along the pathway of the nerves to the spinal cord and then to the brain. Best to call your doctory for a examination to determine the cause. It may be something that can be easily treated with proper care.
C-Spine problem. It could be a pinched nerve eminating from your cervical spine. Typically, from the c3-c4 level. Other than that, it may be a muscle strain. You may want to consult a physician, who should start by taking x-rays of the area. You may require medication and physical therapy to resolve this.