New dog concerns.

Question
We just brought home a aprx. 1 year old pit bull mix from our local shelter.  He has been at the shelter for about 6 months.  He was fixed there about 2 months ago.
Today before we left they gave him a booster shot and microchipped him and a dose of wormer, then we took him to the vet where he had an exam, he had blood drawn for heartworm (neg.) and a rabies shot, then we brought him home.
My question is, I am very suprised that he is not being active, he is just laying around, he has eaten and drank, but mostly he just lays.  Could he be ill?  The vet said he seemed healthy  could it be he is just be he is tired?
Any advice?

P.S. He was not real active at the shelter, but I thought he would sniff out his new surroundings and such.

Answer
It could be it is all too new to him and he is having trouble adjusting, or even a reaction to the rabies shot.  I would let him mellow out.  Give him plenty of attention, but don't force activity.  If he isn't showing more activity by Monday, I would check with the vet.  

He has been through a lot, and it may just be too much for him.