Help. My girlfriend's breasts are becoming larger. I don't know if she's pregnant or is just going through puberty.
Have you talked to her about the changes?
Because that's what it looked like to me.
The best thing to do is talk to her, if you are old enough to have sex with her, you're old enough to ask questions about her body. Just be careful that your phrasing doesn't hurt her feelings and make it clear that you are only asking because you worry about her.