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The swelling in the armpit could simply be some serum or blood filling up in the area (glorified bruise). If this is the case, I would expect to see things improve in the coming week or two. However, I am concerned that there could be more than soft tissue trauma. Because she is young, her growth plates are areas of weakness in the bones, and these spots can fracture a little easier. Because she has nor improved since the fall, I would recommend a vet visit. Your vet could use xrays to determine the extend of the injury. It is important to understand that in young growing animals, bones heal pretty fast. If this is a fracture, you want to have it addressed as soon as possible so prevent a malunion or non union which can lead to a lifetime of complications. There is no over the counter pain med I can recommend at this time as this could limit your vets treatment options.
I hope this helps and gives you direction