Car battery won't start and is newer and has never died before!!?

I just got my truck a couple of months ago from 5 star ford and they had slapped a new battery and everything on. A couple of weeks ago, one of the cables on the battery mysteriously became loose and had to tighten it back on. Just yesterday, went out to the truck, none of the lights were left on and the doors were closed all the way, and the battery was dead. The cord was a little loose again so I tightened it again. I tried jump starting it for about 15 mins, letting it sit and charge. The past week it was giving me a bit of hesitation to start when it would sit for awhile. Could someone give me any advice?? I'm thinking to just buy a new battery but is that really possible or common to have a brand new battery that has never died before to just give out?? I really appreciate any advice.