Only your vet can determine what is actually going on. It could be arthritis or it could by hip dysplasia but you need to consult your vet for the proper diagnosis. They will also be able to prescribe meds that will help your pup with the pain of getting up.

Exercise for the dog is great. Make sure the dog is a good weight. If overweight then the dreaded D word (diet) is highly recommended.

Please don't hesitate to consult your vet for a proper diagnosis. All anyone else can do is guess and that's not in the best interest of your pup!