[20]摸胸女朋友話成長期會痛 點算?

actually are you rubbing her breast too hard?
normally female puberty will ends at about 18 years old
the pain could be by her own or external force
1. when her breasts grow, her skin will stretch which cause her feel pain
2. if being rubbed or squeezed too hard everybody will feel pain if it happens anywhere over the body

i think you should not do this til she is 18

touch her from up to down...
or she lie on you and hate you to touch there...

maybe you can touch her softly.
(less than 0.5kg)
if you dont know what is 0.5kg... find or buy a electronic scale to try what is 0.5kg!

finally you can enjoy your life with your love.

even when she over 18, less than 1kg power to there unless you want her as a slave...

How can a 初中生 do the thing you mansion above?
Focus on study first.