Battery life - did I harm my computer?

I just got my first Mac Air about 2 months ago, but the battery life is already running out really quickly.

The other day I didn't have it connected to a power outlet and I ran it on the battery alone, I had to charge my mobile which I plugged through the USB port, but only for around 5 minutes....and now the battery seem to be dying much quicker than before. Did I harm the battery in any way? I can only get about 3-4 hours of battery life while in the beginning it was around 6-7.
So did I hurt the Mac irreversibly by plugging in the phone for 5 minutes?

i don't think so. i´d recommend to let the battery run completely dry. when the macbook air turns itself out then it´s fine - then recharge it. sometimes you have to "re-configure" the battery - it helps to get it completely empty and then reload.
hope i found the right words as i´m missing some here as a german ;-)