A teen girl might have a full bladder because of her diet. If she consumes a lot of diuretic foods or drinks, like coffee or cranberry juice, it can increase urine production. Also, hormonal changes during puberty can sometimes affect the bladder function. For example, hormonal fluctuations might cause the bladder to be more sensitive or hold less urine than usual. Additionally, if she has a bladder that is anatomically smaller than average, it could feel full more easily.
Well, it could be due to holding her pee for too long. Maybe she was in a situation where there was no accessible toilet, like during a sports event. Sometimes, certain medications can also affect the bladder and make it seem full more often. And if she's nervous or stressed, that can also play a role in how her bladder feels. Stress can cause the muscles around the bladder to tense up, giving the feeling of a full bladder.