Hello, That doesn't sound good. I presume that the fall you sustained following the accident you also might have sustained a musculoskeletal injury in your upper back and chest. This pain aggravates on slight movements. This pain can range from dull aching to severe, associated with fatigue and difficulty in performing daily activities. However it is prudent that we exclude other causes of the pain like fracture of the ribs. The main reason behind this is it will reassure you as well as reduces your anxiety.
So I would suggest you a set of investigations like a chest x ray to rule out any possible fractures and a c spine x ray. You can also relieve the pain by
a) Taking painkillers like ibuprofen twice a day for three days
b) Adequate Bed rest and avoid lifting any heavy objects.
And if the pain is not relieved, then you can consult your physician with the reports for appropriate treatment. Hope you feel better. Thank you.