Truck died

[67Fleet]

Well, I took my truck to the local cruise-in tonight and everything was fine. It got there fine, it started up fine for the cruise fine, then after the cruise I parked it, then when I went to leave the battery chugged once then died. Luckily the guy parked next to me had a booster and we got it going right away. Drove home with no issues. Lights were bright as normal. Pulled it in the garage and put a voltmeter to it with the engine off. 13.4 volts. Tried starting and it barely cranked over. Put the voltmeter to the battery while it was running and it was just over 10 volts. The battery is 4 months old. I don't know how old the alt is but it looks fairly new. What could be the issue?