Windows 8.1 might use up slightly more battery, but this probably is an issue with your battery. Try going into a BIOS setup screen or something separate from Windows, then try unplugging your power cord and see what happens.
If your screen shuts off your battery won't hold any charge at all. Otherwise, it might be detecting the battery is below the 5% automatic hibernate mark, which could be a sign of an inaccurate battery reading. Try this: Go to Control Panel > Power Options > Click "Change plan settings" next to your preferred power plan > Click "Change advanced power settings" > Scroll to bottom and hit "Battery" > Click "Critical battery action" > Change it to "Sleep."
When your computer hits the 5% mark, it should go to sleep instead of hibernation. When it shuts off the screen and goes to sleep completely, power it back on and observe the battery levels. See how long it lasts before it completely shuts off at 0%.
When your computer is going into hibernation the screen shuts off immediately but sometimes mouse and keyboard input is still accepted for about 15-30 seconds before it powers off completely.
Either way, you should try replacing your battery. It shouldn't have anything to do with your upgrade to 8.1. This should just be coincidental. It is possible, but in order to test it you would have to restore to 8.0.