I'm a 16-year-old girl around three months ago I started to receive an odd sensation on the lower left side of my head at the back. Feels pressure and a build-up of fluid. It also conjoins to my left shoulder (unsure dislocation 2013)?

Tension. The symptoms you describe might be from neck muscle tension and that causes pain or fullness in the area. Try relaxing and stretching you neck a few times a day. You can take Ibuprofen for pain if needed. If not better in 2-3 see your primary doctor.