Get a sleeping bag liner. A flannel one will cost about $30 and boost the warmth of your sleeping bag considerably IF you need it.

Pro tip: If you are cold in your bag when you shouldn’t be it probably means you got too hot, started sweating, and now your cold because of evaporation. It took me YEARS to figure that out. I now leave my bag unzipped half way down so I don’t build up moisture inside and that keeps me from getting cold. I’ve comfortably slept in my bag many time in air temperature well below freezing since figuring that out.