My dog has been spazzing out as if he has been bitten, but I cannot see anything or...

[Nikki]

...feel anything on his fur.? My dog for the last couple of hours wipes his paws on the ground, will randomly sniff the air, will then go on to bite at his butt as if he has an itch (same goes for other parts of his body), but I cannot feel or see anything on him. I have brushed him (hoping to get whatever might be on him, off), and I have tried to calm him down, but he hasn't been able to stay still for the last couple of hours. I was thinking fleas at first, but I cannot see any and I gave him his flea medicine about three days ago. My dog is a male, almost 2yrs old, and is a Labrador-Retriever (and possibly Border Collie) mix -- if that might have anything to do with it. He doesn't act as if he is in pain, just bothered/itchy. He hasn't whined at all either. What do do you think could possibly be going on with him??