Part of me thinks, "well duh, she has a crush on you." But, another part remembers being a fat kid who hit puberty and got taller and skinnier. So I understand being that age and having a residual lack of confidence based on past experience. I'm not sure if the mangaka thought it out that far, but well done.
A: sucks B's finger
B: "I wonder why they do this..."
A: licks B's face
B: "I don't understand..."
A: "I wanna fuck you."
B: "... Wait, could they have a crush on- Nah, that's absurd."