I Fell A Week Ago On My Back. It Didn't

I fell a week ago on my back. It didn't hurt and I wasn't bruised. 3 days later i returned to work and my back began to hurt. I have been taking ibuprofen but if I don't stay on top of the pain with the ibuprofen I have a very hard time sleeping or staying comfortable. Is this normal? How long can I expect the pain to last?