I broke my collarbone, and now I am having shoulder pain. Could they be related? I was 10 when I broke my collarbone and now I am 21.

Unlikely. Unless you broke the bone severely and close to the acromion(near the ac joint) the chances of a common mid 1/3 clavicle fx.Causing shoulder pain 10 years later are remote.Common causes of shoulder pain in 21yo females include shoulder instability, tendinitis, referred pain from neck, etc.See your ors if your pain continues.Best of luck!
Clavicle . Clavicle or collar bone fractures can cause shoulder pain if they cause imbalance due to malunion ( when a fracture heals in the wrong way). Ideally new radiographies are needed for a proper evaluation of your case. Shoulder specialists or general ortho surgeons may help you with that. Good luck.