Hit my head about 3 weeks ago and havent had symptoms in a week and last night my head started hurting again and i get dizzy when falling asleep and briefly when i stand up after laying down for a while. could this be something other than concussion?

You did not state how severe the concussion was 3 weeks ago. If it's severe, grade 3 or 4 ( you loss of consciousness), you may need to see your doctor or urgent care because of recurrent symptoms.

Not clear that you had a concussion but did have a closed head injury which can cause post-traumatic headaches, and even vestibular injury to inner ear resulting in postural dizziness. If this is worsening, see a neurologist and get this fully evaluated.