I fell off my horse and for a week now my shoulder and neck have been sore and i'm experiencing daily headaches and some dizziness and feeling off balance. i was wearing a helmet but the back of my head is also slightly sore?

Likely neck: You likely suffered a neck- cervical spine, at least, injury. Having an exam with your primary care doc will help with peace of mind by clarifying diagnosis and will allow for proper treatment.

Evaluation advised: Even after rather trivial head injuries posttraumatic headaches lasting a week or so are not uncommon. However after your accident you're experiencing several symptoms that I feel warrant investigation. Neck & shoulder injuries should be assessed and, to rule out intracranial bleeding & subdural hematoma, I feel a CT scan of the brain should be done. These'll provide you with prognosis & treatment

See your doctor: A thorough direct examination should indeed reassure you, and if some problem is uncovered which needs attention, this will prevent future chronicity.