You should have refused to sign the write-up. Then, called your District union rep to dispute it.
I have always heard the managers only have 7 days after the incident to give a write-up. I have been told recently that they only have three days to give a write-up for attendance.
My humble opinion is they can't give you a write-up for not paying attention.
I scrape my arms and bump my head on the stupid shelf extenders often.
Hopefully you refused to sign the write up. Try to get it revoked nonetheless. Be prepared for "retaliation" if your management is mega douchey. You can NOT be written up for being hurt on the job. Fight it.
Hell I hurt myself 2 times. Each time I tried to file a report & was asked did anyone witness it, I was like no I work in produce in the cooler at night on 3rd shift. I am the only one back there at night. They replied, well since on one witness it cannot file a report on it. So I had to pay for all the medical issues on my back & foot because of this Bull****.