Why did I get under eye bags after upper eyelid surgery?

This should not be a permanent issue for you if only upper eyelid surgery was performed. The swelling on the lower lids can sometimes take weeks to resolve as gravity pulls the swelling down. Lymphatic massage and warm compresses can help reduce the swelling faster. 

Very likely you are experiencing swelling which is moving downwards to lower eyelids due to gravity. You may see some bruising as well. Gentle massage of lower eyelids, low salt, low spices and no MSG diet may help. That should disappear within next few weeks. Good luck.