Breast size

Okay, so I have a question. I'm 17(18 shortly) and I can't tell if my boobs are growing super slow or if I'm just small chested? Out of my family, the smallest boobs are C cups, and I'm a small B right now. They've been this size for about two years, and they still hurt, but I can't tell if it's just period pain or if they're growing? They don't sag but there is a gap between them that's just pure bone and compared to my family I'm the smallest chest. I'm very curvy on the bottom but not super skinny or overweight, so are they still devolving or just small? Thank you ❣️