First, that voltage "before starting the car" at 12.46V means it's about 75% charged -- full charge is at 12.65 when taken after sitting overnight. Maybe because of an alternator problem, and maybe because your battery is just getting old (or sulfated from not being properly recharged regularly).

That "I turned off the car and [the battery] was 13.18V" is useless, because it continues to lower its charge until it's stabilized after many hours.

On a cold morning, 14.5V is very good (the alternator's voltage regulator is set for that in cold weather); "after about five minutes the reading was 14.38" isn't more helpful, but if you checked after the engine ran (and you drove) for a half hour (in cold weather) and found the voltage down to 13.5 or so, that would also be very good -- the regulator is supposed to do that. In hot weather, btw, all these voltages should be about a half volt less.