Wasn't there a scandal publicised a while back whereby these roadside repair people were selling batteries unnecessarily..........even to the point where they were deliberately flattening/draining batteries in order to sell one?
Be wary, yes when it gets colder, it tends to bring to your attention the state of your battery as they have to work harder, but a new one should be fine.
It could also be down to some of the previous mentioned: constant drain i.e boot light, dead cell in the new battery.........take it back, alternator not charging.
