I think this may be another one that we need to add to the boxer trait list. Harley has pulled the same thing with us on several occassions - worried me sick! Same thing - debating "do we take him to the emergency vet - do we wait ???" Harley would hold his leg up so high as if he was getting ready to go or something but we did the same thing - felt his leg and checked in between his paws - nothing. I figured maybe he got bit on his leg by an ant or something. I have learned at this point to always wait until the next morning as he always wakes up a o.k. - they just love the attention 
