Definitely sore muscles from coughing.
The big inhalations needed to produce a cough...as well as the muscles used in the coughing itself add up to thoratic fatigue.
I had either a severely pulled muscle once from a bad cough...or possibly even a minor cracked rib. The pain was suprisingly bad!
I say this not to worry you...but to let you know your sore back is to be expected. (I DO NOT think you've cracked a rib...just letting you know coughing is tough on the body sometimes!)
Hopefully you are able to knock off early and REST!!
Don't do anything when you get home...chores can wait.:)
Rest and recover!!
"Courage is not the abscence of fear, it is feeling afraid and doing it anyway!"
"Worrying does not empty tomorrow of its troubles...it empties today of its strength."
Corrie Ten Boom
Panic Syndrome recovery due to CBT