Dog Started Growling and Snipping?

I recently adopted a 3 yr old Beagle from the humane society. The first few days he was a perfect dog, then he sprained his rear leg playing ball. He wouldn't let us get close to him so we had to wrangle him down to get his leash on him to take him to the ER vet. Ever since sometimes he's happy, but sometimes he growls and tries to bite us if we try to touch him. He really only does it when sitting on my couch or if he's in my son's bed and I try to get him down.

I don't know if he's acting like this because of his leg, (which we stay away from) or if he's becoming possessive over certain places that he lays on. How can I break this? We can't be worried if he's going to snip at us everytime we try to scratch his head or try to get his leash on him.