Your timeline is confusing. You hurt your back at a prior job? And you want to make a claim exclusively against your more recent employer, where you worked for only one week?
You seem to be claiming that you injured your back before you started the job, you made no report of the injury to your employer until the morning you failed to appear, and are trying to recharacterize the complaint you made to your boss ("My back hurts from last week") as a claim for worker's compensation benefits, even though you never formally applied for benefits. Is that correct?
What are the employer's policies relating to sick leave? Did you ever actually complete and submit forms applying for worker's comp benefits? If so, when? What medical documentation did you submit to them that you needed sick leave on July 5?
Your boss probably thought you were hung over from a July 4 bash, and using your back as an excuse to skip work.

