Yep, like @
guybo suggested, have your battery load tested.
Meanwhile, you may consider putting the battery on a charger for a long, slow charge.
Be aware, if a battery has been run down, the alternator will NOT usually bring it back up to speed. The alternator will only replace the electrons that are consumed under normal operation.
Ref:
http://www.aa1car.com/library/battery_runs_down.htm
humfrz