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Greetings, I'm Dr. Peck, a small animal veterinarian in general practice, hopefully I might be of some help, one moment while I write my reply...
With this history I'm going to worry about two things. 1) A Luxating Patella. This is a knee cap that slips out of place causing a lameness like this. Not typically a painful issue but will cause a functional lameness. 2) A torn ACL in the knee. This may cause a similar non-weight bearing or toe-touching lameness in the hind leg.
If you've not become aware of a luxating patella issue at Sophie's age and had her your whole life, then I'm going to be concerned about an ACL tear until a physical examination indicates otherwise. The patella issue is commonly a life-long problem, so you likely would've dealt with it already. ACL tears are common in dogs and wouldn't produce swelling that you'd notice, but often it produces some swelling that a vet can pickup on physical examination.
I wouldn't suggest any pain meds OTC for dogs. I would suggest you rest her overnight, and if she allows it ice her knee for 10-15 minutes. Wrap the ice in a cloth to be sure it's comfortable. Then get her in to your vet during regular hours for an examination.
A physical examination should decipher which of these seems more likely, vs. something else causing the lameness. It is always possible she had a minor sprain and may be improved by tomorrow, but I'd still suggest having her looked at. A luxating patella can be diagnosed easily on examination. Torn ACLs are often suspect by not proven until the dog is put under light sedation and x-rays are obtained to rule out other issues.
Hopefully this advice is of some help to you. If you have further questions don't hesitate to write me back.
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