Hi Kristen,
You didn't say where the sides were sore. Since you mentioned washing
the pad, I thought it might be in that area. I use Orvus Paste horse
shampoo to wash my blankets (I read this somewhere, might have been on
ridecamp) and 2 rinse cycles.
If the sides are sore more where your legs contact the horse, then you
might want to take a look at the blevins buckles to see if they are
smooth or rough. While posting, rough buckles could irritate the horses
sides. These are just some more thoughts to go along with all the other
recommendations, which by the way have been great.
Hope you find the cause and get it corrected real soon.
Dee