Help! Need to pee constantly at bed time?

My almost 4 year old daughter has been crying during every bed and nap time for the last few days saying she has to pee. When she sits on the potty nothing comes out, 5 min later she's demanding to go potty again, resulting in nothing. She has been fully potty trained since 2.5 years old. She has the strongest bladder of any kid i know. She's only had 1 accident ever which was during the day months ago. This only happens at sleep times. During the day she's her normal self. Could it be a bladder infection? I think not since it's only at bed time. Anyone else go through this?