What Causes Headache Followed By Pain On Left Side Of Back?

I was in a rear-end accident in August and my shoulder has been kind of forward since and I had a headache for the rest of the day afterward. Then about two months later I started having pain in the left side of the back of my neck up into the top of my skull and behind my left eye. Some days it s worse than others, and there have been a couple weeks or so when it has gone away on it s own (but continues to come back after a few days of relief) One big factor is sitting with crossed legs or crossing my left ankle over the other, or even standing with my feet together. What could be the underlying issue causing this pain? Any help would be appreciated as constant pain is very exhausting. By the way, ibuprofen does nothing to alleviate the pain, but taking acetaminophen does help.