I didn't have to wear a back brace after my double fusion, but after my redo fusion, my surgeon wanted me to wear one when I went out of the house. I too had a Boston Brace and the first time I wore it for more than 1/2 hour, it hurt me too. I called my surgeon and they told me to just drop in and he would look at it.
Turns out it was not made right and wasn't supporting anything it should have been. If I continued to wear it like that, it would just do more damage than good. He was furisous and call the brace company up right then and there and told them he was sending me over for a new one and they better not charge me. Mind you, this was my second back brace already. The first one the made the day after my surgery was so absurd, even the person who brought it to me the next day knew it was terribly wrong.
I went in to be measured for my third brace and when it came in a few days later, I went in and put it on. Even he knew it wasn't right. He kept going into some back room to work on it and try to make adjustments so it would fit, but after an hour he gave up and said he would have to measure me again and get another one made.
When the 4th one came in and I went in to get it, the second I put it on I knew it was finally right. It felt so good and fit like a glove. Your brace shouldn't hurt you. Have you had your surgeon look at it to make sure it is right? If not, call tomorrow and get it checked right away, you may need another one made too.
Good Luck,
Casey