I would guess its an alternator or belt problem. Test & charge the battery if needed. You could remove your alternator & have it tested for free @ most parts stores, that way you will know for sure. There is some testing you could do yourself, but to do it properly you need expensive equip.
http://www.binghamtonlife.com/manual...s/91-23-72.jpg
http://www.binghamtonlife.com/manual...s/91-23-73.jpg