Have you weighed your bag? How does it compare to the spec weight? The "paper thin" shell / lining won't vary too much in weight. If your bag weigh comes within spec, then you have nothing to worry about. But if your bag weight comes noticeably below -- then you should contact MontBell for exchange.
When I first got my MB #3, it looked and felt so skimpy that I had a lot of doubts about bringing it untested to Wind Rivers. However, actual weight was the same as spec. I ended up bringing the bag, but also brought an extra insulation layer "just in case". To my pleasant surprise, the bag's 32F rating was spot on! Temps were slightly below freezing (we had frost coating on our tents) but I felt comfy throughout -- wearing only silk long johns.