jump car and allow batt to fully charge. shut off car and disconnect batt and let sit overnight (it's possible the batt is taking a dump on you and the cold has just pushed it over the edge). take batt in for test to see if it's dying on it's own-volt and load test(bad cell, etc). if everything checks out then you have an issue w/ the car/wiring/grounding. when it gets cold like this i have seen issues w/ rats getting up in/underneath the batt and dash and chewing on wires. is your car left outside or inside at night?