Guys, I need an advise/opinion.

Last Saturday my wife was driving my X, she stopped for a few minutes and could not start the engine. As she turned the key to "start" position, the radio stopped working and lights shut off. Some truck was passing buy and helped her with a jump start.

Rest of Saturday and the whole Sunday the X was starting just fine. Monday morning it was dead again. No lights, power, radio, nothing. I've called AAA and they gave me a jump start.

Now it works fine again.

The strange facts:
1) The battery is 2years, 2 months old. It's too young to die. (Yeah, I bought it in WalMart for $55. Maybe I just got that I've paid for)
2) Assuming the battery is dead how it could completely loose the charge in few minutes last Saturday, than hold it overnight from Saturday to Sunday?

I just worry that the problem may be not in the battery.

So what do you think - battery or not battery?

PS. I've measured the voltage on the battery with engine running. It's 14.3 volts. So the alternator seems to be working fine.