My Boxer of 16 month's is now limping on her front right paw, this has been going for 2 days now. When she has excercised the leg seems fine, but after a considerable rest the discomfort start and is reluctant to push weight on the leg, is this just muscular?, I have checked between the toes and cannot find anything.
The fact that it is aggravated with rest/lying down for periods and eases up with excercise lead me to think of arthritis for starters. Though that is odd for a dog that young.
I then start thinking her elbows. Were the parents elbows OFA'd, or has this girl had prelims done on them?
A vet appointment will be your only way to determine for sure the cause. It will likely take some investigating and xrays, etc. on the vets part. But he can certainly give her something to help with the discomfort until you can get a diagnosis.