Cedar and Redwood have a toxin in them which kills the bugs that want to invaded then , you are suffering from an overdose of that by sanding them and breathing the dust.
I have warned folks about that on my web site and thought you were aware of it , Guess Not.
To repeat myself........ When sanding , sawing or working with any of those woods make sure you have a good respirator ( Breathing mask ) on and plenty of ventilation or you will have sinus , headaches and other respiratory problems which are NO FUN.
The sap wood is full of them , yep the same wood that makes the boat look so good when it is done.
Even something as simple as carving a small figure from them and then sanding it with an Emory file ( fingernail file ) can do the same to a person , it does not take a lot of the dust to trigger an attack , just depends on the person.