I had a neck pain then a pain in the back of my head (left), then the pain moved to the center. i feel like there's a fluid moving in my head. ?

Muscle spasms: If there was no specific trauma you probably have a migrating muscle spasm that is causing these symptoms. Try an ice pack to the back of your head and non-steroidal anti- inflammatory medication.

Neck pain: Your symptoms are a little vague. Neck pain can be due to many reasons Muscle sprans , cervical spins disorders ot inflammation. We need more specifics in order to help you