Good Morning and welcome to Just answer. I am Dr. Loretta, a licensed veterinarian and I am happy to answer your question.
The most likely cause for this pain is either a partial ruoptureof the Anteriotr Cruciate Ligament or a congenital luxation of the patela or knee cap. For these injuries, it is best to give Mollie aspirin at 5 mg per pound every 12 hrs and keep her as immobile as possible so this can heal.
It would be good to have your vet examine Mollie to be sure she does not have this congenital subluxation of the patella since this could require surgery if it does not improve.
This LINK will give more information and this LINK.
Does that make sense to you?
Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please Reply so I can clarify for you
Sincerely, Dr. Loretta