my car battery died yesterday night.., what happend? Options

[yellowxboy]

yesterday night i was out with my gf .. after dinner we went back to her place to take a nap and when we decided to go out for some coffee, my car wouldn't start. all the lights were very dim, my guages didn't light up, it made a faint clicking noise when i tried turning the key but the motor didnt even move.

at first i assumed the battery died, but i just bought the battery not too long ago (5 months i think). my friends came over to jump start my car and everything is back to normal. my gf's dad thinks that the alternator is old and that i need to buy a new one because it is draining my battery..

any suggestions? i want to get a general idea before i bring it to a machanic to get it fixed.

[ i didnt leave anything on while my car was parked, i have 2 subs that are powered by a 900 watt amp, i rarely turn on my glow lights. but i do drive with my lights on in the morning.. could that be a reason why it drains? ]