"like a month ago" - Less than, more than??
I'd go by best buy (does not have to be the same store) and check with them first, then as jaguarskw indicated call samsung support.
Seems I read something in the manual about periodic recalibratinging the charge. Dought this would be the problem as you should not have problem so quickly. I have the Samsung RF711-SO1. Still able to watch DVD on plane and still have some juice left over. Have had over 6 Months.
Go to Start , Then ALL Programs , Samsung Settings , ECO Mode , And turn the battery saver off !