I have had the same thing happen to my truck three times. The first time I freaked out and pulled over, I noticed my truck was in Regen and took it to the dealer. they advised me it will do this while in regen to clean out the DPF. I have watched it twice since then, the last time I had just pulled out of the car wash and was driving down the road when I realized I couldnt see anyone behind me. I sped up and it continued for almost a block. Asked the diesel tech and he stated they will do this sometimes while in regen.
Hope he is right, but I havent noticed any problems and is running good.