You'd really need a vet for that. A couple of the common reasons are luxating patellas (knee cap slipped out of joint, maintain proper weight and limit things like jumping even on and off the couch and using stairs can help unless your dog is on the bigger side or it's very severe and then you may have to consider surgery), a break, or a torn ligament, sprain, torn toenail, it may also be a bite from something like a spider or infection if there's any swelling. You don't want to mess with it until you know what it is and if it might make it worse. You might also want to put your pup into their kennel and restrict activity before seeing the vet (today is best). Hope your baby is feeling better soon and it's easy to take care of.
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