It is prudent that you take your daughter to an pediatric orthopedist. They specialize in children, who have very different growth and development patterns than adults. It is a sub-specialty and they are very focused, so they will provide accurate information about diagnosis and treatment.
Baker's cysts (or popliteal cysts) are quite common.
They usually are not a primary lesion (develop on their own), but as a result of something else going on in the knee. When there is extra fluid in the knee, or the knee cannot handle the extra fluid it is producing, the fluid then develops into a cyst in the back of the leg.
Only because your daughter should be active and involved in sports should this be looked at. She is at the prime age for so many sports activities and being active in life in general.
While waiting for the appointment see the specialist, give your daughter children's motrin (or ibuprofen in children's doses). It has some anti-inflammatory properties that tylenol doesn't have.
Hope this helps. Take care and best wishes.
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