Since march, I have pain in the left side of my neck. Not all the time. But. Sometimes the pain moves to the back of my head too. What you Think?

Local Myalgia. Local myalgia includes muscle co-contraction and muscle soreness. Good rest, moist heat, passive muscle stretching and OTC NSAIDs will solve the problem. If pain persists after 2 weeks see your doctor for examination and tests.
Muscle spasm. This sounds like muscle spasm of the supporting neck muscles,actually quite common. I suggest using Advil or Aleve (naproxen) , if not allergic and also BioFreeze cram or gel.