For about two weeks, i've had an isolated pain on the top left side of my head, not from injury. what might be causing this?

Scalp sore: If not from injury , consider the possible sources. It could be something in the skin - a lesion or a sore. It could be something deeper than the skin in the muscular or fatty layers . There might be a skull growth , cyst , and lastly something deeper within the brain could be the source. Bottom line is to go see md if you can't exclude something simple and it won't go away