Well for some perspective, if your girlfriend is 20 and really is hitting on a 14-15 year old, that is a much bigger red flag than pretty much anything else in the picture, if your SO is an adult who hits on young teenagers. Like seriously, yikes. So if you truly are suspicious, I would advise turning your focus away from insecurity and jealousy, and turning it instead onto the creepy pedo-vibes.

The other possibility is that your girlfriend is just really kind, and is trying to give a lonely, insecure teenager a sense of friendship, approval and belonging. The teenager herself might harbor some sort of crush, that is fairly typical for teenagers who receive friendship-type attention from an attractive adult. Happens. But it could very well be that your girlfriend is just trying to be a nice person.