Injuries to the back can cause quite a bit of discomfort, just like injuries to other parts of the body.
Physicians usually state, that if you are having numbness, tingling, or electrical shooting type pain, then you probably should be seen. Loss of bowel control is another reason to be seen acutely. If the pain is so significant that you cannot control it with ice/heat and over-the-counter medicines, then you may want to see your physician.
It is also recommended that total rest be limited to as short of a period of time as possible, but definitely no more than 48 hours. The long you are at complete rest the more atrophy you will develop, making it just that much harder to get back to activity.
So, it is usually recommended that you try to do some light activity, walking is a very good light activity.
You will most likely get very sore from the contusion. You may even notice a bruise at the location where you struck the ground. Again, ice is recommended for any acute pain, whereas heat is recommended for its soothing nature.
But, if you have any neurological symptoms or bowel issues, or the pain is just such that you can't control it, then you should see your physician.